Semiconductor device and method for manufacturing the same

ABSTRACT

A semiconductor device in which fluctuation in electric characteristics due to miniaturization is less likely to be caused is provided. The semiconductor device includes an oxide semiconductor film including a first region, a pair of second regions in contact with side surfaces of the first region, and a pair of third regions in contact with side surfaces of the pair of second regions; a gate insulating film provided over the oxide semiconductor film; and a first electrode that is over the gate insulating film and overlaps with the first region. The first region is a CAAC oxide semiconductor region. The pair of second regions and the pair of third regions are each an amorphous oxide semiconductor region containing a dopant. The dopant concentration of the pair of third regions is higher than the dopant concentration of the pair of second regions.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a semiconductor device including anoxide semiconductor and a method for manufacturing the semiconductordevice.

In this specification, the semiconductor device refers to all devicesthat can function by utilizing semiconductor characteristics. Atransistor in this specification is a semiconductor device, and anelectrooptic device, a semiconductor circuit, and an electronic deviceincluding the transistor are all semiconductor devices.

2. Description of the Related Art

Transistors used for most flat panel displays typified by a liquidcrystal display device and a light-emitting display device are formedusing silicon semiconductors such as amorphous silicon, single crystalsilicon, and polycrystalline silicon provided over glass substrates.Further, transistors formed using such silicon semiconductors are usedin integrated circuits (ICs) and the like.

Attention has been directed to a technique in which, instead of theabove silicon semiconductors, metal oxides exhibiting semiconductorcharacteristics are used for transistors. Note that in thisspecification, a metal oxide exhibiting semiconductor characteristics isreferred to as an oxide semiconductor.

For example, a technique is disclosed in which a transistor ismanufactured using zinc oxide or an In—Ga—Zn—O-based oxide as an oxidesemiconductor and the transistor is used as a switching element or thelike of a pixel of a display device (see Patent Documents 1 and 2).

Further, a technique is disclosed in which in a transistor including anoxide semiconductor, a highly conductive oxide semiconductor containingnitrogen is provided as buffer layers between a source region and asource electrode and between a drain region and a drain electrode, andthereby the contact resistance between the oxide semiconductor and thesource electrode and between the oxide semiconductor and the drainelectrode is reduced (see Patent Document 3).

Further, as a method for forming a source region and a drain region of atransistor including an oxide semiconductor in a self-aligned manner, amethod is disclosed in which a surface of the oxide semiconductor isexposed and argon plasma treatment is performed, and thereby theresistivity of the exposed portion of the oxide semiconductor is reduced(see Non-Patent Document 1).

In this method, however, since the surface of the oxide semiconductor isexposed and argon plasma treatment is performed, portions of the oxidesemiconductor to be the source region and the drain region are alsoetched, leading to decrease in the thicknesses of the source region andthe drain region (see FIG. 8 in Non-Patent Document 1). As a result, theresistance of the source region and the drain region is increased, andin addition, defective products are produced with higher probabilityowing to overetching due to the decrease in thickness.

This phenomenon is remarkable in the case where the atomic radius of anion species used for the plasma treatment on the oxide semiconductor islarge.

Such a problem does not arise if an oxide semiconductor layer has asufficient thickness. However, when the channel length is less than orequal to 200 nm, it is necessary that the thickness of a portion of theoxide semiconductor layer which serves as a channel be less than orequal to 20 nm, preferably less than or equal to 10 nm, for preventionof a short-channel effect. The above plasma treatment is not suitable inthe case where such a thin oxide semiconductor layer is used.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2007-123861-   [Patent Document 2] Japanese Published Patent Application No.    2007-096055-   [Patent Document 3] Japanese Published Patent Application No.    2010-135774

Non-Patent Document

-   [Non-Patent Document 1] S. Jeon et al., “180 nm Gate Length    Amorphous InGaZnO Thin Film Transistor for High Density Image Sensor    Application”, IEDM Tech. Dig., p. 504, 2010.

SUMMARY OF THE INVENTION

In an integrated circuit including a transistor, the transistor needs tobe miniaturized to achieve higher integration.

A transistor whose channel length is extremely shortened forminiaturization may have fluctuation in electric characteristics, suchas decrease in the threshold voltage. This phenomenon is called ashort-channel effect, and suppression of the short-channel effect is achallenge for miniaturization of a transistor.

It is known that a transistor including an oxide semiconductorparticularly has small off-state current at room temperature, ascompared with a transistor including silicon. It is considered that thisis because the number of carriers generated by thermal excitation issmall, that is, the carrier density is low.

An object of one embodiment of the present invention is to provide asemiconductor device in which fluctuation in electric characteristicsdue to miniaturization is less likely to be caused.

As a means for achieving the above object, in a transistor including anoxide semiconductor, a region containing a dopant is provided in anoxide semiconductor film including a channel formation region.Specifically, two pairs of amorphous regions each containing a dopantare provided in the oxide semiconductor film including the channelformation region, and the dopant concentration is varied between thepairs of regions. In this manner, an electric field generated in a drainregion of the oxide semiconductor film can relieve an electric fieldapplied to the channel formation region, and thus a short-channel effectcan be suppressed. Note that in this specification, a dopantcollectively refers to elements added to an oxide semiconductor filmincluding a channel formation region.

In addition, the oxide semiconductor of the channel formation region isnon-single-crystal; specifically, the channel formation region includescrystal portion in which atoms are arranged in a triangle, a hexagon, aregular triangle, or a regular hexagon when seen from the directionperpendicular to the a-b plane of the non-single-crystal and in whichmetal atoms or metal atoms and oxygen atoms are arranged in layers whenseen from the direction perpendicular to the c-axis. Note that in thisspecification, such crystal portion is referred to as c-axis alignedcrystal (CAAC) and such oxide semiconductor including the c-axis alignedcrystal is referred to as CAAC oxide semiconductor (CAAC-OS: c-axisaligned crystalline oxide semiconductor). With the channel formationregion is formed as a CAAC oxide semiconductor region, fluctuation inelectric characteristics of the transistor due to irradiation withvisible light or ultraviolet light can be suppressed and the reliabilityof the semiconductor device can be improved.

One embodiment of the present invention is a semiconductor device whichincludes an oxide semiconductor film including a first region, a pair ofsecond regions in contact with side surfaces of the first region, and apair of third regions in contact with side surfaces of the pair ofsecond regions; a gate insulating film provided over the oxidesemiconductor film; and a first electrode that is over the gateinsulating film and overlaps with the first region. The first region isa CAAC oxide semiconductor region. The pair of second regions and thepair of third regions are each an amorphous oxide semiconductor regioncontaining a dopant. The dopant concentration of the pair of thirdregions is higher than the dopant concentration of the pair of secondregions.

The oxide semiconductor film preferably contains two or more elementsselected from In, Ga, Sn, and Zn.

The above semiconductor device further includes a second electrode and athird electrode that are electrically connected to the pair of thirdregions.

The pair of second regions and the pair of third regions can be formedin a self-aligned manner by adding the dopant through the gateinsulating film and a sidewall insulating film provided on side surfacesof the first electrode. In other words, with the sidewall insulatingfilm, the pair of second regions can be regions to which a smalleramount of dopant is added (referred to as low-concentration regions inthis specification). The pair of third regions can be regions to which alarger amount of dopant is added (referred to as high-concentrationregions in this specification). Further, with the sidewall insulatingfilm, the pair of second regions can be provided between the firstregion functioning as a channel formation region and the pair of thirdregions functioning as a source region and a drain region.

The dopant added to the pair of second regions and the pair of thirdregions is hydrogen or one or more elements selected from rare gaselements, and the concentration of the dopant contained in each of thepair of second regions and the pair of third regions is preferablyhigher than or equal to 1×10¹⁹ atoms/cm³ and lower than or equal to1×10²² atoms/cm³. It is further preferable that the dopant concentrationof the pair of second regions be higher than or equal to 5×10¹⁸atoms/cm³ and lower than 5×10¹⁹ atoms/cm³ and that the dopantconcentration of the pair of third regions be higher than or equal to5×10¹⁹ atoms/cm³ and lower than or equal to 1×10²² atoms/cm³.

In the semiconductor device according to one embodiment of the presentinvention, the second electrode and the third electrode may be incontact with either top surfaces of the pair of third regions or bottomsurfaces of the pair of third regions.

An area where the gate insulating film is formed depends on how to formthe sidewall insulating film. Specifically, the gate insulating film canbe formed over the first region, the second regions, and the thirdregions or only over the first region.

In the case where a nitride insulating film is used as the sidewallinsulating film and an oxide insulating film is used as the gateinsulating film, the gate insulating film functions as an etchingstopper in formation of the sidewall insulating film owing to theetching selectivity between the nitride insulator and the oxideinsulator, so that excessive etching of the oxide semiconductor film incontact with a bottom surface of the gate insulating film can besuppressed. As a result, in the semiconductor device having thestructure, the gate insulating film is left over the first region, thepair of second regions, and the pair of third regions.

In the case where an oxide insulating film is used as each of thesidewall insulating film and the gate insulating film, the gateinsulating film provided over the pair of second regions and the pair ofthird regions can be etched by utilizing the etching selectivity betweenthe oxide insulating film and the first electrode. As a result, in thesemiconductor device having the structure, the gate insulating film isleft over the first region.

The addition of the dopant for forming the low-concentration regions andthe high-concentration regions in the transistor which is one embodimentof the present invention can be performed by an ion doping method, anion implantation method, or the like. Furthermore, instead of performingion doping or ion implantation, the dopant can be added by generatingplasma in an atmosphere of a gas containing the dopant added andperforming plasma treatment on an object to which the dopant is added.

In addition, in the case where an element with a large atomic radius,such as a rare gas element, is added as the dopant, it is preferablethat the above plasma treatment be performed with the oxidesemiconductor film covered with the gate insulating film (with the gateinsulating film provided over the first region, the pair of secondregions, and the pair of third regions) for the following reason. In themanufacturing process of the transistor, if the above plasma treatmentis performed with the oxide semiconductor film exposed, portions of theoxide semiconductor film to be the pair of third regions might be etchedand decreased in thickness.

In this manner, the portions of the oxide semiconductor film to be thehigh-concentration regions can be prevented from being etched anddecrease in the thicknesses of the high-concentration regions can besuppressed. In addition, cleanliness of the interface between the oxidesemiconductor film and the gate insulating film can be maintained, andthus the electric characteristics and reliability of the transistor canbe improved.

According to one embodiment of the present invention, a semiconductordevice which includes an oxide semiconductor, has favorable electriccharacteristics and reliability, and is easily miniaturized can beprovided.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B are a top view and a cross-sectional view, respectively,illustrating an example of a semiconductor device which is oneembodiment of the present invention;

FIGS. 2A to 2C illustrate a method for manufacturing a semiconductordevice which is one embodiment of the present invention;

FIGS. 3A to 3E illustrate a method for manufacturing a semiconductordevice which is one embodiment of the present invention;

FIGS. 4A and 4B are a top view and a cross-sectional view, respectively,illustrating an example of a semiconductor device which is oneembodiment of the present invention;

FIGS. 5A to 5E illustrate a method for manufacturing a semiconductordevice which is one embodiment of the present invention;

FIGS. 6A and 6B are a top view and a cross-sectional view, respectively,illustrating an example of a semiconductor device which is oneembodiment of the present invention;

FIGS. 7A to 7E illustrate a method for manufacturing a semiconductordevice which is one embodiment of the present invention;

FIGS. 8A and 8B illustrate band structures of oxide semiconductors and ametal material;

FIGS. 9A to 9D are cross-sectional views illustrating examples of asemiconductor device which is one embodiment of the present invention;

FIGS. 10A and 10B are cross-sectional views illustrating examples of aresistor element which is one embodiment of the present invention;

FIGS. 11A and 11B are examples of circuit diagrams each illustrating oneembodiment of the present invention;

FIG. 12 is an example of a circuit diagram illustrating one embodimentof the present invention;

FIGS. 13A and 13B are examples of circuit diagrams each illustrating oneembodiment of the present invention;

FIGS. 14A and 14B are examples of circuit diagrams each illustrating oneembodiment of the present invention; and

FIG. 15A is a block diagram illustrating a specific example of a CPU,and FIGS. 15B and 15C are circuit diagrams of part thereof.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described in detail withreference to the accompanying drawings. Note that the present inventionis not limited to the description below, and it is easily understood bythose skilled in the art that various changes and modifications can bemade without departing from the spirit and scope of the presentinvention. Therefore, the present invention should not be construed asbeing limited to the description in the following embodiments. Note thatin structures of the present invention described below, the sameportions or portions having similar functions are denoted by the samereference numerals in different drawings, and description thereof is notrepeated.

Note that in each drawing described in this specification, the size, thefilm thickness, or the region of each component is exaggerated forclarity in some cases. Therefore, embodiments of the present inventionare not limited to such scales.

Note that terms such as “first”, “second”, and “third” in thisspecification are used in order to avoid confusion among components, andthe terms do not limit the components numerically. Therefore, forexample, the term “first” can be replaced with the term “second”,“third”, or the like as appropriate.

Functions of a “source” and a “drain” may be replaced with each otherwhen the direction of current flow is changed in circuit operation, forexample. Therefore, the terms “source” and “drain” can be used to denotethe drain and the source, respectively, in this specification.

Embodiment 1

In this embodiment, a structure of a transistor which is one embodimentof the present invention and a method for manufacturing the transistorwill be described with reference to FIGS. A and 1B, FIGS. 2A to 2C, andFIGS. 3A to 3E.

(Structure and Characteristic of Transistor 100)

FIG. 1A is a plan view of a transistor 100. Note that a base insulatingfilm 102, a gate insulating film 111, and an interlayer insulating film117 are not illustrated in FIG. 1A for convenience.

In FIG. 1A, a first electrode 113 and a sidewall insulating film 115 onside surfaces of the first electrode 113 are provided over an oxidesemiconductor film 103. Further, a second electrode 119 a and a thirdelectrode 119 b are provided over a pair of third regions 109 a and 109b in the oxide semiconductor film 103 through openings 116 a and 116 b.The second electrode 119 a and the third electrode 119 b are in contactwith top surfaces of the pair of third regions 109 a and 109 b. Thetransistor 100 is a top-gate top-contact transistor.

FIG. 1B is a cross-sectional view of the transistor 100 along A-B. InFIG. 1B, the base insulating film 102 is provided over a substrate 101,and the oxide semiconductor film 103 including a first region 105, apair of second regions 107 a and 107 b, and the pair of third regions109 a and 109 b is provided over the base insulating film 102. The pairof second regions 107 a and 107 b is provided in contact with sidesurfaces of the first region 105. The pair of third regions 109 a and109 b is provided in contact with side surfaces of the pair of secondregions 107 a and 107 b.

The gate insulating film 111 is provided over the oxide semiconductorfilm 103. The first electrode 113 which overlaps with the first region105 is provided over the gate insulating film 111. Sidewall insulatingfilms 115 a and 115 b (the sidewall insulating film 115) are provided incontact with the side surfaces of the first electrode 113.

The interlayer insulating film 117 is provided over the gate insulatingfilm 111, the first electrode 113, and the sidewall insulating films 115a and 115 b.

The second electrode 119 a and the third electrode 119 b are provided incontact with the pair of third regions 109 a and 109 b through theopening 116 a and 116 b provided in the gate insulating film 111 and theinterlayer insulating film 117. Note that the gate insulating film 111is in contact with the first region 105, the pair of second regions 107a and 107 b, and the pair of third regions 109 a and 109 b.

Although end portions of the second electrode 119 a and the thirdelectrode 119 b may be tapered, the first electrode 113 preferably has avertical end. The first electrode 113 is formed to have a vertical end,an insulating film to be the sidewall insulating film 115 (the sidewallinsulating films 115 a and 115 b) is formed over the first electrode113, and highly anisotropic etching is performed; thus, the sidewallinsulating film 115 (the sidewall insulating films 115 a and 115 b) canbe formed.

In FIGS. 1A and 1B, the pair of second regions 107 a and 107 bcorresponds to regions where the oxide semiconductor film 103 overlapswith the sidewall insulating film 115, which will be described in detaillater. Further, at least part of the sidewall insulating film 115 (thesidewall insulating films 115 a and 115 b) may be curved except forregions in contact with the side surfaces of the first electrode 113 andthe gate insulating film 111.

The oxide semiconductor film 103 including the first region 105, thepair of second regions 107 a and 107 b, and the pair of third regions109 a and 109 b is a metal oxide containing two or more elementsselected from In, Ga, Sn, and Zn. Note that the metal oxide has abandgap greater than or equal to 2 eV, preferably greater than or equalto 2.5 eV, further preferably greater than or equal to 3 eV. Theoff-state current of the transistor 100 can be reduced by using such ametal oxide having a wide bandgap.

In the transistor 100, the first region 105 functions as a channelformation region.

The first region 105 is the CAAC oxide semiconductor region describedabove. The CAAC oxide semiconductor is not single crystal, but this doesnot mean that the CAAC is composed of only an amorphous component.Although the CAAC oxide semiconductor includes a crystallized portion (acrystalline portion), a boundary between one crystalline portion andanother crystalline portion is not clear in some cases. Nitrogen may besubstituted for part of oxygen included in the CAAC oxide semiconductor.The c-axes of individual crystalline portions included in the CAAC oxidesemiconductor may be aligned in one direction (e.g., a directionperpendicular to a surface of a substrate over which the CAAC oxidesemiconductor is formed or a surface, a film surface, an interface, orthe like of the CAAC oxide semiconductor film). Alternatively, thenormals of the a-b planes of the individual crystalline portionsincluded in the CAAC oxide semiconductor may be aligned in one direction(e.g., a direction perpendicular to the substrate surface over which theCAAC oxide semiconductor is formed or the surface, the film surface, theinterface, or the like of the CAAC oxide semiconductor film). Note thatthe CAAC oxide semiconductor can be a conductor, a semiconductor, or aninsulator, depending on the composition or the like. Further, the CAACoxide semiconductor transmits or does not transmit visible light,depending on the composition or the like. As an example of the CAACoxide semiconductor, a material is given in which triangular orhexagonal atomic arrangement can be observed from the directionperpendicular to a surface of the deposited material, a surface of asubstrate over which the material is deposited, or an interface of thedeposited material and in which layered arrangement of metal atoms orlayered arrangement of metal atoms and oxygen atoms (or nitrogen atoms)can be observed in a cross section of the deposited material.

The hydrogen concentration of the first region 105 is lower than orequal to 1×10²⁰ atoms/cm³, preferably lower than or equal to 1×10¹⁹atoms/cm³, further preferably lower than or equal to 1×10¹⁸ atoms/cm³.The transistor 100 in which the first region 105 serving as the channelformation region is the CAAC oxide semiconductor region and the hydrogenconcentration is reduced is a highly reliable transistor having stableelectric characteristics, because change in the threshold voltagebetween before and after light irradiation and a gate bias-temperature(BT) stress test is small.

The pair of second regions 107 a and 107 b and the pair of third regions109 a and 109 b each have a conductivity higher than or equal to 10 S/cmand lower than or equal to 1000 S/cm, preferably higher than or equal to100 S/cm and lower than or equal to 1000 S/cm. Further, the conductivityof the pair of third regions 109 a and 109 b is higher than theconductivity of the pair of second regions 107 a and 107 b. Note thatwhen the conductivity is too low, the on-state current of the transistor100 is decreased.

In addition, the pair of second regions 107 a and 107 b and the pair ofthird regions 109 a and 109 b are each an amorphous region containing adopant. Hydrogen or one or more elements selected from rare gas elementsare added to the pair of second regions 107 a and 107 b and the pair ofthird regions 109 a and 109 b as the dopant.

The carrier density can be increased when the dopant concentrations ofthe pair of second regions 107 a and 107 b and the pair of third regions109 a and 109 b are increased; however, an excessively high dopantconcentration causes the dopant to inhibit transfer of carriers,resulting in decrease in the conductance of the pair of second regions107 a and 107 b and the pair of third regions 109 a and 109 b.

Therefore, it is preferable that the pair of second regions 107 a and107 b and the pair of third regions 109 a and 109 b each have a dopantconcentration higher than or equal to 5×10¹⁸ atoms/cm³ and lower than orequal to 1×10²² atoms/cm³. Further, the dopant concentration of the pairof third regions 109 a and 109 b is higher than the dopant concentrationof the pair of second regions 107 a and 107 b. Specifically, it ispreferable that the dopant concentration of the pair of second regions107 a and 107 b be higher than or equal to 5×10¹⁸ atoms/cm³ and lowerthan 5×10¹⁹ atoms/cm³ and that the dopant concentration of the pair ofthird regions 109 a and 109 b be higher than or equal to 5×10¹⁹atoms/cm³ and lower than or equal to 1×10²² atoms/cm³. In addition, sucha difference in the dopant concentration is made in a self-alignedmanner in a step of adding the dopant, because the sidewall insulatingfilm 115 (the sidewall insulating films 115 a and 115 b) is provided inthe transistor 100.

The pair of third regions 109 a and 109 b functions as a source regionand a drain region of the transistor 100. In the transistor 100,amorphous regions having different dopant concentrations(low-concentration regions and high-concentration regions) are providedat both ends of the first region 105 serving as the channel formationregion, whereby an electric field applied to the first region 105serving as the channel formation region can be relieved. Specifically,the pair of second regions 107 a and 107 b serving as thelow-concentration regions and the pair of third regions 109 a and 109 bserving as the high-concentration regions are provided at both the endsof the first region 105 serving as the channel formation region, wherebyan effect in which a band edge of a channel formed in the first region105 is hardly curved is exhibited in the transistor 100. Accordingly,provision of the pair of second regions 107 a and 107 b and the pair ofthird regions 109 a and 109 b can suppress a short-channel effect.

(Method for Manufacturing Transistor 100)

Next, a method for manufacturing the transistor 100 will be describedwith reference to FIGS. 2A to 2C and FIGS. 3A to 3E.

The base insulating film 102 is formed over the substrate 101. The baseinsulating film 102 can be formed by a sputtering method, a CVD method,a coating method, or the like. Note that the thickness of the baseinsulating film 102 is preferably, but not limited to, 50 nm or more.

There is no particular limitation on a material and the like of thesubstrate 101 as long as the material has heat resistance high enough towithstand at least heat treatment performed later. For example, a glasssubstrate, a ceramic substrate, a quartz substrate, or a sapphiresubstrate may be used as the substrate 101. Alternatively, a singlecrystal semiconductor substrate or a polycrystalline semiconductorsubstrate made of silicon, silicon carbide, or the like, a compoundsemiconductor substrate made of silicon germanium or the like, an SOIsubstrate, or the like may be used as the substrate 101. Furtheralternatively, any of these substrates provided with a semiconductorelement may be used as the substrate 101.

A flexible substrate may also be used as the substrate 101. In the casewhere a transistor is provided over the flexible substrate, thetransistor may be formed directly on the flexible substrate, or thetransistor may be formed over a different substrate and then separatedfrom the substrate to be transferred to the flexible substrate. In orderto separate the transistor from the substrate and transfer it to theflexible substrate, a region which is easily separated is preferablyprovided between the different substrate and the transistor.

The base insulating film 102 prevents diffusion of an impurity (e.g., analkali metal such as Li or Na) from the substrate 101 and etching of thesubstrate 101 in an etching step in a manufacturing process of thetransistor 100.

The base insulating film 102 is formed to have a single-layer structureor a staked-layer structure using any of insulating films selected fromoxide insulating films such as a silicon oxide film, a gallium oxidefilm, and an aluminum oxide film; nitride insulating films such as asilicon nitride film and an aluminum nitride film; a silicon oxynitridefilm; an aluminum oxynitride film; and a silicon nitride oxide film.Note that the base insulating film 102 preferably contains oxygen in aportion in contact with the oxide semiconductor film 103.

In the case of being formed by a sputtering method, the base insulatingfilm 102 may be formed using a silicon target, a quartz target, analuminum target, an aluminum oxide target, or the like in an atmospheregas containing oxygen. The proportion of oxygen in the atmosphere gas is6 vol. % or higher, preferably 50 vol. % or higher, to the wholeatmosphere gas. By increasing the proportion of the oxygen gas in theatmosphere gas, an insulating film from which oxygen is released byheating can be formed.

Hydrogen in the target is preferably removed as much as possible.Specifically, an oxide target including an OH group at 100 ppm or lower,preferably 10 ppm or lower, further preferably 1 ppm or lower is used,whereby the hydrogen concentration in the base insulating film 102 canbe reduced and thus the electric characteristics and reliability of thetransistor 100 can be improved. For example, fused quartz is preferablebecause it is easily formed so as to include an OH group at 10 ppm orlower and is inexpensive. Needless to say, a target of synthetic quartzhaving a low OH group concentration may be used.

Furthermore, in the manufacture of the transistor 100, the content of analkali metal such as Li or Na, which is an impurity, is preferably low.In the case where a glass substrate containing an impurity such as analkali metal is used as the substrate 101, the above nitride insulatingfilm is preferably formed as the base insulating film 102 in order toprevent entry of an alkali metal. It is further preferable to stack theabove oxide insulating film over the nitride insulating film.

In this specification, silicon oxynitride refers to a substance thatcontains more oxygen than nitrogen and for example, silicon oxynitrideincludes oxygen, nitrogen, silicon, and hydrogen at concentrationsranging from greater than or equal to 50 at. % and less than or equal to70 at. %, greater than or equal to 0.5 at. % and less than or equal to15 at. %, greater than or equal to 25 at. % and less than or equal to 35at. %, and greater than or equal to 0 at. % and less than or equal to 10at. %, respectively. Further, silicon nitride oxide refers to asubstance that contains more nitrogen than oxygen and for example,silicon nitride oxide includes oxygen, nitrogen, silicon, and hydrogenat concentrations ranging from greater than or equal to 5 at. % and lessthan or equal to 30 at. %, greater than or equal to 20 at. % and lessthan or equal to 55 at. %, greater than or equal to 25 at. % and lessthan or equal to 35 at. %, and greater than or equal to 10 at. % andless than or equal to 25 at. %, respectively. Note that the above rangesare obtained by measurement using Rutherford backscattering spectrometry(RBS) or hydrogen forward scattering spectrometry (HFS). In addition,the total of the percentages of the constituent elements does not exceed100 at. %.

In addition, since the base insulating film 102 preferably containsoxygen in a portion in contact with the oxide semiconductor film 103, aninsulating film from which oxygen is released by heating may be used asthe base insulating film 102. Note that the expression “oxygen isreleased by heating” means that the amount of released oxygen which isconverted into oxygen atoms is greater than or equal to 1.0×10¹⁸atoms/cm³, preferably greater than or equal to 3.0×10²⁰ atoms/cm³, inthermal desorption spectroscopy (TDS) analysis.

A method for quantifying the amount of released oxygen which isconverted into oxygen atoms, with the use of TDS analysis will bedescribed below.

The amount of released gas in TDS analysis is proportional to theintegral value of a spectrum. Therefore, the amount of released gas canbe calculated from the ratio of the integral value of a spectrum of aninsulating film to the reference value of a standard sample. Thereference value of a standard sample refers to the ratio of the densityof a predetermined atom contained in a sample to the integral value of aspectrum.

For example, the number of released oxygen molecules (NO₂) from aninsulating film can be found according to Numerical Expression 1 withthe TDS analysis results of a silicon wafer containing hydrogen at apredetermined density which is the standard sample and the TDS analysisresults of the insulating film. Here, all spectra having a mass numberof 32 which are obtained by the TDS analysis are assumed to originatefrom an oxygen molecule. CH₃OH, which is given as a gas having a massnumber of 32, is not taken into consideration on the assumption that itis unlikely to be present. Further, an oxygen molecule including anoxygen atom having a mass number of 17 or 18 which is an isotope of anoxygen atom is not taken into consideration either because theproportion of such a molecule in the natural world is minimal.[FORMULA 1]N_(O2)=N_(H2)/S_(H2)×S_(O2)×α  (Numerical Expression 1)

N_(H2) is the value obtained by conversion of the number of hydrogenmolecules desorbed from the standard sample into density. S_(H2) is theintegral value of a spectrum of the standard sample which is analyzed byTDS. Here, the reference value of the standard sample is set toN_(H2)/S_(H2). S_(O2) is the integral value of a spectrum of theinsulating film which is analyzed by TDS. α is a coefficient whichinfluences spectrum intensity in TDS analysis. Japanese Published PatentApplication No. H6-275697 can be referred to for details of NumericalExpression 1. Note that the above value of the amount of released oxygenis obtained by measurement with a thermal desorption spectroscopyapparatus produced by ESCO Ltd., EMD-WA1000S/W using a silicon wafercontaining hydrogen atoms at 1×10¹⁶ atoms/cm³ as the standard sample.

Further, in the TDS analysis, part of oxygen is detected as an oxygenatom. The ratio between oxygen molecules and oxygen atoms can becalculated from the ionization rate of the oxygen molecules. Note that,since the above a includes the ionization rate of the oxygen molecules,the number of the released oxygen atoms can also be estimated throughthe evaluation of the number of the released oxygen molecules.

Note that N_(O2) is the number of the released oxygen molecules. For theinsulating film, the amount of released oxygen in the case of beingconverted into oxygen atoms is twice the number of the released oxygenmolecules.

As an example of the insulating film from which oxygen is released byheating, oxygen-excess silicon oxide (SiO_(X) (X>2)) is given. In theoxygen-excess silicon oxide (SiO_(X) (X>2)), the number of oxygen atomsper unit volume is more than twice the number of silicon atoms per unitvolume. The number of silicon atoms and the number of oxygen atoms perunit volume are measured by Rutherford backscattering spectrometry.

By using the insulating film from which oxygen is released by heating asthe base insulating film 102, oxygen can be supplied to the oxidesemiconductor film 103 and interface states between the base insulatingfilm 102 and the oxide semiconductor film 103 can be reduced.Accordingly, electric charge or the like that can be generated owing tooperation of the transistor 100 can be prevented from being trapped atthe interface between the base insulating film 102 and the oxidesemiconductor film 103, and thus the transistor 100 can be a transistorwith little deterioration of electric characteristics.

Further, electric charge is generated owing to an oxygen vacancy in theoxide semiconductor film 103 in some cases. In general, when oxygenvacancies are caused in an oxide semiconductor, part of the oxygenvacancies becomes a donor to generate an electron as a carrier. That is,also in the transistor 100, part of oxygen vacancies in the oxidesemiconductor film 103 becomes a donor to generate an electron as acarrier and thus the threshold voltage of the transistor 100 isnegatively shifted. In addition, the generation of an electron in theoxide semiconductor film 103 often occurs in oxygen vacancies caused inthe vicinity of the interface between the oxide semiconductor film 103and the base insulating film 102. When oxygen is sufficiently releasedfrom the base insulating film 102 to the oxide semiconductor film 103,oxygen vacancies in the oxide semiconductor film 103, which might causethe negative shift of the threshold voltage, can be compensated.

That is, by using the insulating film from which oxygen is released byheating as the base insulating film 102, interface states between theoxide semiconductor film 103 and the base insulating film 102 and oxygenvacancies in the oxide semiconductor film 103 can be reduced; thus, aninfluence of charge trap at the interface between the oxidesemiconductor film 103 and the base insulating film 102 can be reduced.

Next, the oxide semiconductor film 103 is formed over the baseinsulating film 102.

Specifically, an oxide semiconductor film 140 which is an entirely CAACoxide semiconductor film is formed, and then a dopant is added to theoxide semiconductor film 140, so that the pair of second regions 107 aand 107 b and the pair of third regions 109 a and 109 b are formed. Inthis manner, the oxide semiconductor film 103 is formed. Here, a methodfor forming the oxide semiconductor film 140 which includes CAAC in thestate before the dopant is added to form the pair of second regions 107a and 107 b and the pair of third regions 109 a and 109 b will bedescribed.

There are two methods for forming the oxide semiconductor film 140 whichis a CAAC oxide semiconductor film.

One of the methods is a method in which an oxide semiconductor isdeposited while a substrate is heated (referred to as a 1-step methodfor convenience), and the other method is a method in which an oxidesemiconductor is deposited twice and heat treatment is performed twice(referred to as a 2-step method for convenience).

Firstly, a method for forming the oxide semiconductor film 140 on thebasis of the 1-step method will be described.

First, the oxide semiconductor material given in the description of theoxide semiconductor film 103 is deposited by a sputtering method whilethe substrate 101 provided with the base insulating film 102 is heated.Note that an oxide semiconductor film formed in this step is referred toas an oxide semiconductor film 130 for convenience. The temperature atwhich the substrate 101 is heated may be higher than or equal to 200° C.and lower than or equal to 400° C., preferably higher than or equal to250° C. and lower than or equal to 350° C. The oxide semiconductor film130 may be formed to a thickness greater than or equal to 1 nm and lessthan or equal to 50 nm.

Here, a sputtering apparatus used for formation of the oxidesemiconductor film 130 will be described in detail below.

The leakage rate of a treatment chamber in which the oxide semiconductorfilm 130 is formed is preferably lower than or equal to 1×10⁻¹⁰ Pa·m³/s;thus, entry of an impurity into the film can be suppressed in theformation by a sputtering method.

In order to lower the leakage rate, internal leakage as well as externalleakage needs to be reduced. The external leakage refers to inflow ofgas from the outside of a vacuum system through a minute hole, a sealingdefect, or the like. The internal leakage is due to leakage through apartition, such as a valve, in a vacuum system or due to released gasfrom an internal member. Measures need to be taken from both aspects ofexternal leakage and internal leakage in order that the leakage rate belower than or equal to 1×10⁻¹⁰ Pa·m³/s.

In order to reduce external leakage, an open/close portion of thetreatment chamber is preferably sealed with a metal gasket. For themetal gasket, a metal material covered with iron fluoride, aluminumoxide, or chromium oxide is preferably used. The metal gasket realizeshigher adhesion than an O-ring, and can reduce the external leakage.Further, by use of a metal material covered with iron fluoride, aluminumoxide, chromium oxide, or the like which is in the passive state,released gas containing hydrogen generated from the metal gasket issuppressed, so that the internal leakage can also be reduced.

As a member for an inner wall of the treatment chamber, aluminum,chromium, titanium, zirconium, nickel, or vanadium, from which a gascontaining hydrogen is less likely to be released, or an alloy materialwhich contains at least one of iron, chromium, nickel, and the like andis covered with any of these elements may be used. The alloy materialcontaining at least one of iron, chromium, nickel, and the like isrigid, resistant to heat, and suitable for processing. Here, whensurface unevenness of the member is reduced by polishing or the like toreduce the surface area of the inner wall of the treatment chamber, thereleased gas can be reduced. Alternatively, the member may be coveredwith iron fluoride, aluminum oxide, chromium oxide, or the like which isin the passive state.

Furthermore, it is preferable to provide a refiner for an atmosphere gasjust in front of the treatment chamber. At this time, the length of apipe between the refiner and the treatment chamber is less than or equalto 5 m, preferably less than or equal to 1 m. When the length of thepipe is less than or equal to 5 m or less than or equal to 1 m, aninfluence of the released gas from the pipe can be reduced accordingly.

Evacuation of the treatment chamber is preferably performed with a roughvacuum pump, such as a dry pump, and a high vacuum pump, such as asputter ion pump, a turbo molecular pump, or a cryopump, in appropriatecombination. The turbo molecular pump has an outstanding capability inevacuating a large-sized molecule, whereas it has a low capability inevacuating hydrogen or water. Hence, combination of a cryopump having ahigh capability in evacuating water and a sputter ion pump having a highcapability in evacuating hydrogen is effective.

An adsorbate present in the treatment chamber does not affect thepressure in the treatment chamber because it is adsorbed on the innerwall, but the adsorbate leads to release of gas at the time of theevacuation of the treatment chamber. Therefore, although the leakagerate and the evacuation rate do not have a correlation, it is importantthat the adsorbate present in the treatment chamber be desorbed as muchas possible and evacuation be performed in advance with the use of apump having high evacuation capability. Note that the treatment chambermay be subjected to baking for promotion of desorption of the adsorbate.By the baking, the rate of desorption of the adsorbate can be increasedabout tenfold. The baking may be performed at a temperature higher thanor equal to 100° C. and lower than or equal to 450° C. At this time,when the adsorbate is removed while an inert gas is introduced, the rateof desorption of water or the like, which is difficult to desorb only byevacuation, can be further increased.

In a sputtering method, an RF power supply device, an AC power supplydevice, a DC power supply device, or the like can be used as appropriateas a power supply device for generating plasma.

As a target used for forming the oxide semiconductor film 130 by asputtering method, a metal oxide target containing zinc can be used.Alternatively, a metal oxide target containing two or more elementsselected from indium, gallium, tin, and zinc can be used. As the target,for example, any of the following targets can be used: a four-componentmetal oxide such as an In—Sn—Ga—Zn-based metal oxide; three-componentmetal oxides such as an In—Ga—Zn-based metal oxide, an In—Sn—Zn-basedmetal oxide, an In—Al—Zn-based metal oxide, a Sn—Ga—Zn-based metaloxide, an Al—Ga—Zn-based metal oxide, a Sn—Al—Zn-based metal oxide, anIn—Hf—Zn-based metal oxide, an In—La—Zn-based metal oxide, anIn—Ce—Zn-based metal oxide, an In—Pr—Zn-based metal oxide, anIn—Nd—Zn-based metal oxide, an In—Sm—Zn-based metal oxide, anIn—Eu—Zn-based metal oxide, an In—Gd—Zn-based metal oxide, anIn—Tb—Zn-based metal oxide, an In—Dy—Zn-based metal oxide, anIn—Ho—Zn-based metal oxide, an In—Er—Zn-based metal oxide, anIn—Tm—Zn-based metal oxide, an In—Yb—Zn-based metal oxide, and anIn—Lu—Zn-based metal oxide; two-component metal oxides such as anIn—Zn-based metal oxide, a Sn—Zn-based metal oxide, and an In—Ga-basedmetal oxide; and a single-component metal oxide containing indium, tin,zinc, or the like.

As an example of the target, a metal oxide target containing In, Ga, andZn (an In—Ga—Zn-based metal oxide) has a composition ratio whereIn₂O₃:Ga₂O₃:ZnO=1:1:1 [molar ratio]. Alternatively, a target having acomposition ratio where In₂O₃:Ga₂O₃:ZnO=1:1:2 [molar ratio], a targethaving a composition ratio where In₂O₃:Ga₂O₃:ZnO=1:1:4 [molar ratio], ora target having a composition ratio where In₂O₃:Ga₂O₃:ZnO=2:1:8 [molarratio] can be used.

As the atmosphere gas, a rare gas (typically argon) atmosphere, anoxygen atmosphere, or a mixed gas of a rare gas and oxygen is used asappropriate. It is preferable that a high-purity gas from whichimpurities such as hydrogen, water, hydroxyl, and hydride are removed beused as the atmosphere gas.

With the use of the above sputtering apparatus, the oxide semiconductorfilm 130 into which entry of hydrogen is suppressed can be formed.

The base insulating film 102 and the oxide semiconductor film 130 may besuccessively formed in vacuum. For example, after impurities includinghydrogen over the surface of the substrate 101 are removed by heattreatment or plasma treatment, the base insulating film 102 may beformed without exposure to the air, and the oxide semiconductor film 130may be successively formed without exposure to the air. In this manner,impurities including hydrogen over the surface of the substrate 101 canbe reduced, and an atmospheric component can be prevented from attachingto the interface between the substrate 101 and the base insulating film102 and the interface between the base insulating film 102 and the oxidesemiconductor film 130. As a result, it is possible to manufacture thetransistor 100 having favorable electric characteristics and highreliability.

Then, a resist mask is formed over the oxide semiconductor film 130 in afirst photolithography step. Processing is performed using the resistmask in a first etching step, so that an island-shaped oxidesemiconductor film 132 is formed. Note that the resist mask can beformed by an ink-jet method, a printing method, or the like asappropriate, as well as through the photolithography step.

In the first etching step, etching is preferably performed so that anend portion of the island-shaped oxide semiconductor film 132 istapered. The island-shaped oxide semiconductor film 132 is formed tohave a tapered end portion, whereby the coverage with the gateinsulating film 111 formed later can be improved. In the case of using aphotolithography step, the tapered shape can be obtained by etchingwhile removing the resist mask.

The first etching step may be dry etching, wet etching, or combinationthereof. As an etchant used for wet etching, a mixed solution ofphosphoric acid, acetic acid, and nitric acid, an ammonia hydrogenperoxide mixture (31 wt % hydrogen peroxide water: 28 wt % ammoniawater:water=5:2:2 (volume ratio)), or the like can be used.Alternatively, ITO07N (produced by KANTO CHEMICAL CO., INC.) may beused.

As an etching gas for dry etching, a gas containing chlorine (achlorine-based gas such as chlorine (Cl₂), boron trichloride (BCl₃),silicon tetrachloride (SiCl₄), or carbon tetrachloride (CCl₄)) ispreferably used.

Alternatively, a gas containing fluorine (a fluorine-based gas such ascarbon tetrafluoride (CF₄), sulfur hexafluoride (SF₆), nitrogentrifluoride (NF₃), or trifluoromethane (CHF₃)); hydrogen bromide (HBr);oxygen (O₂); any of these gases to which a rare gas such as helium (He)or argon (Ar) is added; or the like can be used.

For dry etching, a parallel plate reactive ion etching (RIE) method oran inductively coupled plasma (ICP) etching method can be used. In orderto process the film into a desired shape, the etching condition (theamount of electric power applied to a coil-shaped electrode, the amountof electric power applied to an electrode on a substrate side, thetemperature of the electrode on the substrate side, or the like) isadjusted as appropriate.

Heat treatment is performed after the oxide semiconductor film 132 isformed, so that the oxide semiconductor film 140 is formed. The heattreatment is performed at a temperature higher than or equal to 150° C.and lower than or equal to 650° C., preferably higher than or equal to250° C. and lower than or equal to 450° C., in an oxidation atmosphereor an inert atmosphere. Here, the oxidation atmosphere refers to anatmosphere including an oxidation gas such as oxygen, ozone, or nitrogenoxide at 10 ppm or higher. The inert atmosphere refers to an atmospherewhich includes the oxidation gas at lower than 10 ppm and is filled withnitrogen or a rare gas. The treatment time is 3 minutes to 24 hours. Theratio of a crystalline region to an amorphous region in the oxidesemiconductor film can be increased as the treatment time is increased.Note that heat treatment for longer than 24 hours is not preferablebecause the productivity is decreased. Note that the heat treatment maybe performed after formation of the oxide semiconductor film 132 and thegate insulating film 111.

By the heat treatment, hydrogen is released from the oxide semiconductorfilm 132, and in addition, part of oxygen contained in the baseinsulating film 102 is diffused to the oxide semiconductor film 132 anda portion of the base insulating film 102, which is in the vicinity ofthe interface with the oxide semiconductor film 132.

There is no particular limitation on a heat treatment apparatus used forthe heat treatment, and the apparatus may be provided with a device forheating an object to be processed by heat radiation or heat conductionfrom a heating element such as a resistance heating element. Forexample, an electric furnace, or a rapid thermal annealing (RTA)apparatus such as a gas rapid thermal annealing (GRTA) apparatus or alamp rapid thermal annealing (LRTA) apparatus can be used. An LRTAapparatus is an apparatus for heating an object to be processed byradiation of light (an electromagnetic wave) emitted from a lamp such asa halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arclamp, a high pressure sodium lamp, or a high pressure mercury lamp. AGRTA apparatus is an apparatus for heat treatment using ahigh-temperature gas.

Here, a method for forming the oxide semiconductor film 140 on the basisof the 2-step method will be described.

A first oxide semiconductor film is formed, and first heat treatment isperformed at a temperature higher than or equal to 400° C. and lowerthan or equal to 750° C. in an atmosphere of nitrogen, oxygen, a raregas, or dry air. By the first heat treatment, a first crystalline oxidesemiconductor film having a crystalline region is formed in a regionincluding a surface of the first oxide semiconductor film. Then, asecond oxide semiconductor film which is thicker than the first oxidesemiconductor film is formed, and second heat treatment is performed ata temperature higher than or equal to 400° C. and lower than or equal to750° C., so that crystal growth proceeds upward with the use of thefirst crystalline oxide semiconductor film as a seed for the crystalgrowth and the whole second oxide semiconductor film is crystallized (asecond crystalline oxide semiconductor film is formed). The firstcrystalline oxide semiconductor film and the second crystalline oxidesemiconductor film, which are formed in the above-described manner, areused as the oxide semiconductor film 130, the first photolithographystep and the first etching step are performed so that the oxidesemiconductor film 132 is formed, and the heat treatment performed afterthe formation of the oxide semiconductor film 132 in the 1-step methodis performed; thus, the oxide semiconductor film 140 can be formed. Notethat a heat treatment apparatus used for the first heat treatment andthe second heat treatment may be any of the heat treatment apparatuseswhich can be used for the heat treatment performed after the formationof the oxide semiconductor film 132 in the 1-step method.

Next, the gate insulating film 111 and the first electrode 113 areformed over the oxide semiconductor film 140 (see FIG. 3A). The gateinsulating film 111 can be formed in a manner similar to that of thebase insulating film 102. The thickness of the gate insulating film 111is preferably greater than or equal to 1 nm and less than or equal to300 nm, further preferably greater than or equal to 5 nm and less thanor equal to 50 nm.

The gate insulating film 111 is formed to have a single-layer structureor a stacked-layer structure using any of insulating films selected froma silicon oxide film, a gallium oxide film, an aluminum oxide film, asilicon nitride film, a silicon oxynitride film, an aluminum oxynitridefilm, and a silicon nitride oxide film. It is preferable that the gateinsulating film 111 also contain oxygen in a portion in contact with theoxide semiconductor film 103. Alternatively, an insulating film fromwhich oxygen is released by heating may be used. The insulating filmfrom which oxygen is released by heating is used as the gate insulatingfilm 111, whereby a defect caused in the oxide semiconductor film 103can be repaired and deterioration of electric characteristics of thetransistor 100 can be suppressed.

A high-k material such as hafnium oxide, yttrium oxide, hafnium silicate(HfSi_(x)O_(y) (x>0,y>0)), hafnium silicate to which nitrogen is added(HfSi_(x)O_(y)N_(z) (x>0,y>0,z>0)), or hafnium aluminate (HfAl_(x)O_(y)(x>0,y>0)) may be used. Because of high dielectric constant, a high-kmaterial enables increase in the physical thickness of the gateinsulating film while maintaining the capacitance of the gate insulatingfilm as the same as the case that, for example, a silicon film is usedfor the gate insulating film, thereby reducing the gate leakage current.Note that the gate insulating film 111 may have a single-layer structureusing the high-k material or a stacked-layer structure using a film ofthe high-k material and the above insulating film.

For the first electrode 113, a conductive film is formed using any ofthe above conductive materials by a sputtering method. A secondphotolithography step is performed to form a resist mask over theconductive film, and then the conductive film is processed using theresist mask in a second etching step; thus, the first electrode 113 isformed. The thickness of the first electrode 113 is not particularlylimited and can be determined as appropriate in consideration of theelectric resistance of the conductive material used and time for theformation step.

Further, it is preferable that the gate insulating film 111 and theconductive film to be the first electrode 113 be successively formedwithout exposure to the air.

The first electrode 113 is formed to have a single-layer structure or astacked-layer structure including, as a conductive material, any ofmetals such as aluminum, titanium, chromium, nickel, copper, yttrium,zirconium, molybdenum, silver, tantalum, and tungsten and an alloycontaining any of these metals as a main component. For example, asingle-layer structure of an aluminum film containing silicon, atwo-layer structure in which a titanium film is stacked over an aluminumfilm, a two-layer structure in which a titanium film is stacked over atungsten film, a two-layer structure in which a copper film is formedover a copper-magnesium-aluminum alloy film, and a three-layer structurein which a titanium film, an aluminum film, and a titanium film arestacked in this order can be given. Note that a transparent conductivematerial containing indium oxide, tin oxide, or zinc oxide may be used.Note that the first electrode 113 also functions as a wiring.

Further, an In—Ga—Zn—O film containing nitrogen, an In—Sn—O filmcontaining nitrogen, an In—Ga—O film containing nitrogen, an In—Zn—Ofilm containing nitrogen, a Sn—O film containing nitrogen, an In—O filmcontaining nitrogen, or a film of a metal nitride (such as InN or ZnN)is preferably provided between the first electrode 113 and the gateinsulating film 111. These films each have a work function of 5 eV orhigher, preferably 5.5 eV or higher, and thus the threshold voltage inelectric characteristics of the transistor 100 can be positivelyshifted; consequently, the transistor 100 can be a so-callednormally-off transistor. For example, in the case of using an In—Ga—Zn—Ofilm containing nitrogen, an In—Ga—Zn—O film having a higher nitrogenconcentration than at least the oxide semiconductor film 140,specifically, an In—Ga—Zn—O film having a nitrogen concentration of 7at. % or higher is used.

Next, the sidewall insulating films 115 a and 115 b are formed. Thesidewall insulating film 115 (including the sidewall insulating films115 a and 115 b) is formed using any of the insulating films given inthe description of the base insulating film 102 and the gate insulatingfilm 111.

In the transistor 100, the gate insulating film 111 is provided over allof the first region 105, the pair of second regions 107 a and 107 b, andthe pair of third regions 109 a and 109 b. In order to obtain such astructure, the gate insulating film 111 and the sidewall insulating film115 (including the sidewall insulating films 115 a and 115 b) may beformed using insulating films having different etching rates. With sucha structure, the gate insulating film 111 can function as an etchingstopper in formation of the sidewall insulating film 115. By using thegate insulating film 111 as an etching stopper, excessive etching of theoxide semiconductor film 140 can be suppressed. Moreover, an end pointof the etching for forming the sidewall insulating film 115 can beeasily detected. In addition, when the gate insulating film 111functions as an etching stopper, the width of the sidewall insulatingfilm 115 (the widths of portions where the sidewall insulating films 115a and 115 b are in contact with the gate insulating film 111 in FIG. 1B)can be easily controlled. The area of the pair of second regions 107 aand 107 b serving as the low-concentration regions is determined inaccordance with the width of the sidewall insulating film 115 (thewidths of the portions where the sidewall insulating films 115 a and 115b are in contact with the gate insulating film 111 in FIG. 1B). As thearea of the low-concentration regions is increased, an electric fieldapplied to the first region 105 functioning as the channel formationregion can be further relieved.

First, an insulating film 114 to be the sidewall insulating films 115 aand 115 b is formed over the gate insulating film 111 and the firstelectrode 113 (see FIG. 3B). The insulating film 114 can be formed in amanner similar to that of the base insulating film 102 and is formedusing any of the insulating films listed above. There is no particularlimitation on the thickness of the insulating film 114. The insulatingfilm 114 is subjected to a third etching step, so that the sidewallinsulating films 115 a and 115 b are formed (see FIG. 3C). The thirdetching step is highly anisotropic etching, and the sidewall insulatingfilms 115 a and 115 b can be formed in a self-aligned manner byperforming the highly anisotropic etching step on the insulating film114. Here, dry etching is preferably employed as the highly anisotropicetching, and a gas containing fluorine such as trifluoromethane (CHF₃),octafluorocyclobutane (C₄F₈), or tetrafluoromethane (CF₄) can be used asan etching gas, for example. A rare gas such as helium (He) or argon(Ar) or hydrogen (H₂) may be added to the etching gas. In addition, asthe dry etching, a reactive ion etching method (an RIE method) in whichhigh-frequency voltage is applied to a substrate is preferably used.

Further, the dopant concentration of the pair of second regions 107 aand 107 b depends on the thicknesses of the sidewall insulating films115 a and 115 b; therefore, the thicknesses of the sidewall insulatingfilms 115 a and 115 b and the thickness of the first electrode 113 maybe determined so that the dopant concentration of the pair of secondregions 107 a and 107 b is within the above range. Note that thethickness of the sidewall insulating film 115 a or 115 b here means thelength from a plane thereof which is in contact with the gate insulatingfilm 111 to the highest point of a plane thereof which is in contactwith the first electrode 113.

In addition, the area of the pair of second regions 107 a and 107 bserving as the low-concentration regions is determined in accordancewith the width of the sidewall insulating film 115 (here, the widths ofthe portions where the sidewall insulating films 115 a and 115 b are incontact with the gate insulating film 111 in FIG. 1B). Furthermore, thewidth of the sidewall insulating film 115 depends on the thickness ofthe first electrode 113; therefore, the thickness of the first electrode113 may be determined so that the pair of second regions 107 a and 107 bhas a desired area.

Next, treatment for adding a dopant 150 to the oxide semiconductor film140 is performed, so that the oxide semiconductor film 103 is formed(see FIG. 3D).

The dopant 150 added is hydrogen or one or more elements selected fromrare gas elements. As a method for adding the dopant 150 to the oxidesemiconductor film 140, an ion doping method or an ion implantationmethod can be used. When an ion doping method or an ion implantationmethod is used, the depth to which the dopant 150 is added (an additionregion) can be easily controlled and thus the dopant 150 can be added tothe oxide semiconductor film 140 with high accuracy. The dopant 150 maybe added by an ion doping method or an ion implantation method while thesubstrate 101 is heated. Furthermore, instead of performing ion dopingor ion implantation, the dopant can be added by generating plasma in anatmosphere of a gas containing the dopant added and performing plasmatreatment on an object to which the dopant is added.

Hydrogen serves as an electron donor in the oxide semiconductor film 140and causes the oxide semiconductor film 140 to have n-type conductivity.A rare gas element forms a defect in the oxide semiconductor film 140and causes the oxide semiconductor film 140 to have n-type conductivity.Since hydrogen easily diffuses, diffusion of hydrogen to the firstregion 105 serving as the channel formation region might causedeterioration of transistor characteristics. For this reason, a rare gaselement is preferably used as the dopant 150.

In addition, in the case where an element with a large atomic radius,such as a rare gas element, is added as the dopant 150, it is preferablethat the above plasma treatment be performed with the gate insulatingfilm provided over the first region, the pair of second regions, and thepair of third regions. For example, in the transistor 100, if the aboveplasma treatment is performed with the pair of third regions 109 a and109 b serving as the source region and the drain region exposed,portions of the oxide semiconductor film 140 to be the pair of thirdregions 109 a and 109 b might be etched and decreased in thickness. Byperforming the plasma treatment with the gate insulating film 111provided over the first region 105, the pair of second regions 107 a and107 b, and the pair of third regions 109 a and 109 b, the gateinsulating film 111 can prevent etching of the portions of the oxidesemiconductor film 140 to be the pair of third regions 109 a and 109 band suppress the decrease in thickness. In addition, cleanliness of theinterface between the oxide semiconductor film 103 and the gateinsulating film 111 can be maintained, and thus the electriccharacteristics and reliability of the transistor 100 can be improved.

In the addition of the dopant 150 to the oxide semiconductor film 140,the dopant 150 is added to the oxide semiconductor film 140 through thegate insulating film 111 and the sidewall insulating films 115 a and 115b. Further, in the oxide semiconductor film 140, the amount of the addeddopant 150 is smaller in a region to which the dopant 150 is addedthrough the gate insulating film 111 and the sidewall insulating film115 a or 115 b than in a region to which the dopant 150 is added throughonly the gate insulating film 111. Thus, the pair of second regions 107a and 107 b and the pair of third region 109 a and 109 b are formed in aself-aligned manner (see FIG. 3E). Note that the dopant 150 is not addedto a region of the oxide semiconductor film 140, which overlaps with thefirst electrode 113.

Further, the pair of second regions 107 a and 107 b and the pair ofthird regions 109 a and 109 b get lower crystallinity because of damagedue to the addition of the dopant 150 and become amorphous regions. Notethat by adjusting the additive amount of the dopant 150 or the like, thedegree of damage can be reduced so that the pair of second regions 107 aand 107 b and the pair of third regions 109 a and 109 b are preventedfrom becoming completely amorphous. In that case, the pair of secondregions 107 a and 107 b and the pair of third regions 109 a and 109 beach have at least a higher proportion of an amorphous region than thefirst region 105.

Further, heat treatment may be performed after the dopant 150 is added.The heat treatment may be performed in a manner similar to that of theheat treatment performed in the formation of the oxide semiconductorfilm 140, and is preferably performed at a temperature at which the pairof second regions 107 a and 107 b and the pair of third regions 109 aand 109 b are not crystallized.

Note that the treatment for adding the dopant 150 to the oxidesemiconductor film 140 may be performed plural times. In the case wherethe treatment for adding the dopant 150 to the oxide semiconductor film140 is performed plural times, the kind of the dopant 150 may be thesame in the plural treatments or different in every treatment. Forexample, treatment may be performed in the following order: the firstelectrode 113 is formed as in FIG. 3A, treatment for adding the dopant150 (first addition treatment) is performed, the sidewall insulatingfilms 115 a and 115 b are formed, and treatment for adding the dopant150 (second addition treatment) is performed. The dopant 150 may be thesame element or different elements in the first addition treatment andthe second addition treatment.

Next, an insulating film to be the interlayer insulating film 117 isformed over the gate insulating film 111, the sidewall insulating films115 a and 115 b, and the first electrode 113, and a thirdphotolithography step and a fourth etching step are performed on theinsulating film and the gate insulating film 111, so that the openings116 a and 116 b are formed. The third photolithography step and thefourth etching step may be similar to the first photolithography stepand the first etching step.

The interlayer insulating film 117 may be formed using a silicon oxidefilm, a silicon oxynitride film, a silicon nitride oxide film, or asilicon nitride film by a sputtering method, a CVD method, or the like.At this time, a film from which oxygen is less likely to be released byheating is preferably used as the interlayer insulating film 117 inorder to prevent decrease in the conductivity of the pair of secondregions 107 a and 107 b and the pair of third regions 109 a and 109 b.Specifically, the interlayer insulating film 117 may be formed by a CVDmethod with the use of a mixture which includes a silane gas as a mainmaterial and a proper source gas selected from a nitrogen oxide gas, anitrogen gas, a hydrogen gas, and a rare gas. In addition, the substratetemperature may be higher than or equal to 300° C. and lower than orequal to 550° C. By using a CVD method, the film from which oxygen isless likely to be released by heating can be formed.

Next, the second electrode 119 a and the third electrode 119 b areformed to be connected to the pair of third regions 109 a and 109 bthrough the openings 116 a and 116 b (see FIG. 1B).

Each of the second electrode 119 a and the third electrode 119 b alsofunctions as a wiring and is formed using any of the materials given inthe description of the first electrode 113.

In the transistor 100, the pair of third regions 109 a and 109 b incontact with the second electrode 119 a and the third electrode 119 b isregions having high conductivity, to which the dopant is added;therefore, the contact resistance between the second electrode 119 a andthe third region 109 a and between the third electrode 119 b and thethird region 109 b can be reduced. Accordingly, the on-state current ofthe transistor 100 can be increased.

The second electrode 119 a and the third electrode 119 b are formed insuch a manner that a conductive film is formed using any of the aboveconductive materials as in the case of the first electrode 113 and isthen subjected to a fourth photolithography step and a fifth etchingstep. Note that the fourth photolithography step and the fifth etchingstep may be similar to the first photolithography step and the firstetching step.

Through the above steps, the transistor 100 can be manufactured.

As described above, according to one embodiment of the disclosedinvention, a problem due to miniaturization can be solved. As a result,the size of the transistor can be sufficiently reduced. When the size ofthe transistor is sufficiently reduced, the area occupied by asemiconductor device is also reduced and thus the number ofsemiconductor devices manufactured from one substrate is increased.Accordingly, manufacturing cost of the semiconductor device is reduced.Further, the size of the semiconductor device can be reduced with itsfunction maintained; therefore, the semiconductor device can haveimproved functions as compared with a conventional one of the same size.Further, effects such as high-speed operation, low power consumption,and the like can be obtained because of reduction in channel length.Thus, miniaturization of a transistor including an oxide semiconductorcan be achieved according to one embodiment of the disclosed invention,and various effects accompanied with the miniaturization can beobtained. Note that this embodiment can be combined with any of theother embodiments as appropriate.

Embodiment 2

In this embodiment, a transistor 200 having a structure which is partlydifferent from the structure of the transistor 100 described inEmbodiment 1 will be described.

(Structure and Characteristic of Transistor 200)

The transistor 200 is a transistor which includes a gate insulating filmhaving a shape different from that of the gate insulating film 111 ofthe transistor 100.

FIG. 4A is a plan view of the transistor 200. Note that a baseinsulating film 202, a gate insulating film 211, and an interlayerinsulating film 217 are not illustrated in FIG. 4A for convenience.

In FIG. 4A, a first electrode 213 and a sidewall insulating film 215 onside surfaces of the first electrode 213 are provided over an oxidesemiconductor film 203. Further, a second electrode 219 a and a thirdelectrode 219 b are provided over third regions 209 a and 209 b in theoxide semiconductor film 203 through openings 216 a and 216 b. Thesecond electrode 219 a and the third electrode 219 b are in contact withtop surfaces of the third regions 209 a and 209 b. The transistor 200 isa top-gate top-contact transistor.

FIG. 4B is a cross-sectional view of the transistor 200 along C-D. InFIG. 4B, the base insulating film 202 is provided over a substrate 201,and the oxide semiconductor film 203 including a first region 205, apair of second regions 207 a and 207 b, and the pair of third regions209 a and 209 b is provided over the base insulating film 202. The pairof second regions 207 a and 207 b is provided in contact with sidesurfaces of the first region 205. The pair of third regions 209 a and209 b is provided in contact with side surfaces of the pair of secondregions 207 a and 207 b.

The gate insulating film 211 is provided over the oxide semiconductorfilm 203. The gate insulating film 211 is in contact with the firstregion 205. The first electrode 213 which overlaps with the first region205 is provided over the gate insulating film 211. Sidewall insulatingfilms 215 a and 215 b are provided in contact with the side surfaces ofthe first electrode 213.

The second electrode 219 a and the third electrode 219 b are in contactwith the top surfaces of the pair of third regions 209 a and 209 bthrough the openings 216 a and 216 b in the interlayer insulating film217 which is provided over the first electrode 213 and the sidewallinsulating films 215 a and 215 b.

Although end portions of the second electrode 219 a and the thirdelectrode 219 b may be tapered, the first electrode 213 preferably has avertical end. The first electrode 213 is formed to have a vertical end,an insulating film to be the sidewall insulating film 215 (the sidewallinsulating films 215 a and 215 b) is formed over the first electrode213, and highly anisotropic etching is performed; thus, the sidewallinsulating film 215 (the sidewall insulating films 215 a and 215 b) canbe formed.

In FIG. 4A, the second regions 207 a and 207 b correspond to regionswhere the oxide semiconductor film 203 overlaps with the sidewallinsulating film 215. Further, at least part of the sidewall insulatingfilm 215 is curved except for regions in contact with the side surfacesof the first electrode 213 and the gate insulating film 211.

In the transistor 100, since the gate insulating film 111 is in contactwith the first region 105, the pair of second regions 107 a and 107 b,and the pair of third regions 109 a and 109 b, the openings 116 a and116 b are provided in the gate insulating film 111 and the interlayerinsulating film 117. In contrast, in the transistor 200, since the gateinsulating film 211 is only in contact with the first region 205, theopenings 216 a and 216 b are provided only in the interlayer insulatingfilm 217.

In the transistor 200, the gate insulating film 211 is in contact withthe first region 205, and the gate insulating film 211 does not followthe shape of (does not cover a step formed by) the oxide semiconductorfilm 203. In other words, the gate insulating film 211 does not have aportion which extends beyond a step formed by the oxide semiconductorfilm 203. The gate insulating film 211 does not have a portion whichextends beyond a step formed by the oxide semiconductor film 203, whichcontributes to reduction in the leakage current of the transistor 200due to the gate insulating film 211 and increase in the withstandvoltage of the gate insulating film 211. Therefore, even when the gateinsulating film 211 whose thickness is reduced to around 5 nm is used,the transistor can operate. Note that reduction in the thickness of thegate insulating film 211 leads to suppression of a short-channel effectand increase in the operation speed of the transistor.

Moreover, in the transistor 200, since the gate insulating film 211 doesnot have a portion which extends beyond a step, parasitic capacitance ishardly caused between the first electrode 213 and the pair of secondregions 207 a and 207 b and between the first electrode 213 and the pairof third regions 209 a and 209 b. Consequently, even when the channellength of the transistor 200 is shortened, fluctuation in the thresholdvoltage can be reduced.

(Method for Manufacturing Transistor 200)

Next, a method for manufacturing the transistor 200 will be describedwith reference to FIGS. 2A to 2C and FIGS. 5A to 5E.

In the method for manufacturing the transistor 200, steps preceding astep of forming the insulating film 210 which becomes the gateinsulating film 211 (steps up to and including a step of forming theoxide semiconductor film 140 in FIGS. 2A to 2C) are the same as those ofthe transistor 100; thus, Embodiment 1 can be referred to (see FIGS. 2Ato 2C). Note that the substrate 201 and the base insulating film 202 mayhave the same structures as the substrate 101 and the base insulatingfilm 102 described in Embodiment 1.

Next, an insulating film 210 is formed over the oxide semiconductor film140. The insulating film 210 is formed using a material which can beused for the gate insulating film 111 in Embodiment 1. Then, aconductive film 212 to be the first electrode 213 is formed over theinsulating film 210 (see FIG. 5A). The conductive film 212 is formedusing a conductive material which can be used for the first electrode113 described in Embodiment 1. Note that as a method for forming theconductive film 212, a sputtering method may be used as in Embodiment 1.

Further, it is preferable that the insulating film 210 and theconductive film 212 be successively formed without exposure to the air.

The insulating film 210 and the conductive film 212 are processed, sothat the gate insulating film 211 and the first electrode 213 areformed. By this processing, the gate insulating film 211 which has ashape different from that of the gate insulating film 111 of thetransistor 100 can be formed. Note that the insulating film 210 and theconductive film 212 may be processed using the photolithography step andthe etching step described in Embodiment 1 as appropriate. The thicknessof the gate insulating film 211 may be determined as appropriate on thebasis of the description in Embodiment 1.

Next, an insulating film 214 to be the sidewall insulating films 215 aand 215 b is formed over the oxide semiconductor film 140, the gateinsulating film 211, and the first electrode 213 (see FIG. 5B). Theinsulating film 214 is formed using a material which can be used for thebase insulating film 102 in Embodiment 1. After that, the insulatingfilm 214 is processed, so that the sidewall insulating films 215 a and215 b are formed (see FIG. 5C). A method for processing the insulatingfilm 214 into the sidewall insulating films 215 a and 215 b may be thesame as the method for processing the insulating film 114 into thesidewall insulating films 115 a and 115 b, which is described inEmbodiment 1.

The thickness of the sidewall insulating film 215 a or 215 b means thelength from a plane thereof which is in contact with the oxidesemiconductor film 140 to be the oxide semiconductor film 203 later tothe highest point of a plane thereof which is in contact with the firstelectrode 213. Further, the dopant concentration of the pair of secondregions 207 a and 207 b formed later depends on the thicknesses of thesidewall insulating films 215 a and 215 b; therefore, the thicknesses ofthe sidewall insulating films 215 a and 215 b and the thickness of thefirst electrode 213 may be determined so that the dopant concentrationof the pair of second regions 207 a and 207 b is within the rangedescribed in Embodiment 1.

In addition, the area of the pair of second regions 207 a and 207 bserving as low-concentration regions is determined in accordance withthe width of the sidewall insulating film 215 (here, the widths ofportions where the sidewall insulating films 215 a and 215 b are incontact with the oxide semiconductor film 203 in FIG. 4B). As the areaof the low-concentration regions is increased, an electric field appliedto the first region 205 functioning as a channel formation region can befurther relieved. The width of the sidewall insulating film 215 dependson the thickness of the first electrode 213; therefore, the thickness ofthe first electrode 213 may be determined so that the pair of secondregions 207 a and 207 b has a desired area.

Next, treatment for adding the dopant 150 to the oxide semiconductorfilm 140 is performed (see FIG. 5D). The treatment for adding the dopant150 to the oxide semiconductor film 140 may be performed as inEmbodiment 1. By this treatment, the first region 205, the pair ofsecond regions 207 a and 207 b, and the pair of third regions 209 a and209 b are formed (see FIG. 5E). Note that the first region 205, the pairof second regions 207 a and 207 b, and the pair of third regions 209 aand 209 b which are formed by this treatment have structures similar tothose of the first region 105, the pair of second regions 107 a and 107b, and the pair of third regions 109 a and 109 b described in Embodiment1.

Further, the dopant 150 can be added by a method other than an injectionmethod such as an ion doping method or an ion implantation method. Forexample, plasma treatment can be given in which plasma is generated inan atmosphere of a gas containing the dopant and an object to which thedopant is added (here, the oxide semiconductor film 140) is irradiatedwith the plasma. As an apparatus for generating the plasma, a dryetching apparatus, a plasma CVD apparatus, a high-density plasma CVDapparatus, or the like can be used. Note that the plasma treatment maybe performed while the substrate 201 is heated.

In the case where portions of the oxide semiconductor film 140 to be thepair of third regions 209 a and 209 b are exposed as in the transistor200, if a rare gas element is added as the dopant by plasma treatment,the portions to be the pair of third regions 209 a and 209 b might beetched and decreased in thickness as described in Embodiment 1.Therefore, in the case where the portions of the oxide semiconductorfilm 140 to be the pair of third regions 209 a and 209 b are exposed,hydrogen is preferably used as the dopant.

Note that the treatment for adding the dopant 150 to the oxidesemiconductor film 140 can be performed plural times as in Embodiment 1.

Further, heat treatment may be performed after the dopant 150 is added.The heat treatment may be performed in a manner similar to that of theheat treatment performed in the formation of the oxide semiconductorfilm 140, and is preferably performed at a temperature at which the pairof second regions 207 a and 207 b and the pair of third regions 209 aand 209 b are not crystallized.

The interlayer insulating film 217, the openings 216 a and 216 b, thesecond electrode 219 a, and the third electrode 219 b may be formed inmanners similar to those of the interlayer insulating film 117, theopenings 116 a and 116 b, the second electrode 119 a, and the thirdelectrode 119 b described in Embodiment 1. Through the above steps, thetransistor 200 can be manufactured (see FIG. 4B).

The transistor 200 described in this embodiment can achieve an effectsimilar to that in Embodiment 1. Note that this embodiment can becombined with any of the other embodiments as appropriate.

Embodiment 3

In this embodiment, a transistor 300 having a structure which is partlydifferent from the structure of the transistor described in the aboveembodiment will be described.

(Structure and Characteristic of Transistor 300)

Which surfaces of a second electrode and a third electrode are incontact with a pair of third regions differs between the transistor 300and the transistor 200.

FIG. 6A is a plan view of the transistor 300. Note that a baseinsulating film 302, a gate insulating film 311, and an interlayerinsulating film 317 are not illustrated in FIG. 6A for convenience.

In FIG. 6A, a first electrode 313 and a sidewall insulating film 315 onside surfaces of the first electrode 313 are provided over an oxidesemiconductor film 303. Further, a second electrode 319 a and a thirdelectrode 319 b are in contact with bottom surfaces of third regions 309a and 309 b in the oxide semiconductor film 303. The transistor 300 is atop-gate bottom-contact transistor.

FIG. 6B is a cross-sectional view of the transistor 300 along E-F. InFIG. 6B, the base insulating film 302 is provided over a substrate 301,and the oxide semiconductor film 303 including a first region 305, apair of second regions 307 a and 307 b, and the pair of third regions309 a and 309 b, the second electrode 319 a, and the third electrode 319b are provided over the base insulating film 302. The pair of secondregions 307 a and 307 b is provided in contact with side surfaces of thefirst region 305. The pair of third regions 309 a and 309 b is providedin contact with side surfaces of the pair of second regions 307 a and307 b.

The gate insulating film 311 is provided over the oxide semiconductorfilm 303. The gate insulating film 311 is in contact with the firstregion 305. The first electrode 313 which overlaps with the first region305 is provided over the gate insulating film 311. Sidewall insulatingfilms 315 a and 315 b are provided in contact with the side surfaces ofthe first electrode 313.

The interlayer insulating film 317 is provided over the gate insulatingfilm 311, the first electrode 313, and the sidewall insulating films 315a and 315 b.

Although end portions of the second electrode 319 a and the thirdelectrode 319 b may be tapered, the first electrode 313 preferably has avertical end. The first electrode 313 is formed to have a vertical end,an insulating film to be the sidewall insulating film 315 (the sidewallinsulating films 315 a and 315 b) is formed over the first electrode313, and highly anisotropic etching is performed; thus, the sidewallinsulating film 315 (the sidewall insulating films 315 a and 315 b) canbe formed.

In FIG. 6A, the second regions 307 a and 307 b correspond to regionswhere the oxide semiconductor film 303 overlaps with the sidewallinsulating film 315. Further, at least part of the sidewall insulatingfilm 315 is curved except for regions in contact with the side surfacesof the first electrode 313 and the gate insulating film 311.

In the transistor 300, the gate insulating film 311 is in contact withthe first region 305, and the gate insulating film 311 does not followthe shape of (does not cover a step formed by) the oxide semiconductorfilm 303. In other words, the gate insulating film 311 does not have aportion which extends beyond a step formed by the oxide semiconductorfilm 303. The gate insulating film 311 does not have a portion whichextends beyond a step formed by the oxide semiconductor film 303, whichcontributes to reduction in the leakage current of the transistor 300due to the gate insulating film 311 and increase in the withstandvoltage of the gate insulating film 311. Therefore, even when the gateinsulating film 311 whose thickness is reduced to around 5 nm is used,the transistor can operate. Note that reduction in the thickness of thegate insulating film 311 leads to suppression of a short-channel effectand increase in the operation speed of the transistor.

Moreover, in the transistor 300, since the gate insulating film 311 doesnot have a portion which extends beyond a step, parasitic capacitance ishardly caused between the first electrode 313 and the pair of secondregions 307 a and 307 b and between the first electrode 313 and the pairof third regions 309 a and 309 b. Consequently, even when the channellength of the transistor 300 is shortened, fluctuation in the thresholdvoltage can be reduced.

The transistor 300 illustrated in FIGS. 6A and 6B has a structure inwhich the gate insulating film 311 is provided only in a region incontact with the first electrode 313; however, the gate insulating film311 may also be provided over the third regions 309 a and 309 b (andfurther over the second electrode 319 a and the third electrode 319 b)as in Embodiment 1.

(Method for Manufacturing Transistor 300)

Next, a method for manufacturing the transistor 300 will be describedwith reference to FIGS. 7A to 7E.

The base insulating film 302 is formed over the substrate 301; then, aconductive film to be the second electrode 319 a and the third electrode319 b is formed over the base insulating film 302 and the conductivefilm is processed, so that the second electrode 319 a and the thirdelectrode 319 b are formed. The substrate 301 and the base insulatingfilm 302 may have the same structures as the substrate 101 and the baseinsulating film 102 described in Embodiment 1. The conductive film isformed using a conductive material which can be used for the secondelectrode 119 a and the third electrode 119 b described in Embodiment 1.Note that as a method for forming the conductive film, a sputteringmethod may be used as in Embodiment 1. In addition, the conductive filmmay be processed using the photolithography step and the etching stepdescribed in Embodiment 1 as appropriate.

An oxide semiconductor film 340 is formed over the base insulating film302, the second electrode 319 a, and the third electrode 319 b (see FIG.7A). The oxide semiconductor film 340 can be formed in a manner similarto that of the oxide semiconductor film 140 described in Embodiment 1(see FIGS. 2A to 2C).

Next, the gate insulating film 311 and the first electrode 313 areformed over the second electrode 319 a, the third electrode 319 b, andthe oxide semiconductor film 340. First, an insulating film to be thegate insulating film 311 is formed over the oxide semiconductor film340. The gate insulating film 311 and the first electrode 313 may beformed in manners similar to those of the gate insulating film 211 andthe first electrode 213 in Embodiment 2.

Next, an insulating film 314 to be the sidewall insulating films 315 aand 315 b is formed over the oxide semiconductor film 340, the gateinsulating film 311, and the first electrode 313 (see FIG. 7B). Theinsulating film 314 is formed using a material which can be used for thebase insulating film 102 in Embodiment 1. After that, the insulatingfilm 314 is processed, so that the sidewall insulating films 315 a and315 b are formed (see FIG. 7C). A method for processing the insulatingfilm 314 into the sidewall insulating films 315 a and 315 b may be thesame as the method for processing the insulating film 114 into thesidewall insulating films 115 a and 115 b, which is described inEmbodiment 1.

The thickness of the sidewall insulating film 315 a or 315 b means thelength from a plane thereof which is in contact with the oxidesemiconductor film 340 to be the oxide semiconductor film 303 later tothe highest point of a plane thereof which is in contact with the firstelectrode 313. Further, the dopant concentration of the pair of secondregions 307 a and 307 b formed later depends on the thicknesses of thesidewall insulating films 315 a and 315 b; therefore, the thicknesses ofthe sidewall insulating films 315 a and 315 b and the thickness of thefirst electrode 313 may be determined so that the dopant concentrationof the pair of second regions 307 a and 307 b is within the rangedescribed in Embodiment 1.

In addition, the area of the pair of second regions 307 a and 307 bserving as low-concentration regions is determined in accordance withthe width of the sidewall insulating film 315 (here, the widths ofportions where the sidewall insulating films 315 a and 315 b are incontact with the oxide semiconductor film 340 in FIG. 6B). As the areaof the low-concentration regions is increased, an electric field appliedto the first region 305 functioning as a channel formation region can befurther relieved. The width of the sidewall insulating film 315 dependson the thickness of the first electrode 313; therefore, the thickness ofthe first electrode 313 may be determined so that the pair of secondregions 307 a and 307 b has a desired area.

Next, treatment for adding the dopant 150 to the oxide semiconductorfilm 340 is performed (see FIG. 7D). The treatment for adding the dopant150 to the oxide semiconductor film 340 may be performed as inEmbodiment 1. By this treatment, the first region 305, the pair ofsecond regions 307 a and 307 b, and the pair of third regions 309 a and309 b are formed (see FIG. 7E). Note that the first region 305, the pairof second regions 307 a and 307 b, and the pair of third regions 309 aand 309 b which are formed by this treatment have structures similar tothose of the first region 105, the pair of second regions 107 a and 107b, and the pair of third regions 109 a and 109 b described in Embodiment1.

Further, as in the case of the transistor 200, the transistor 300 has astructure in which the dopant 150 is added with part of the oxidesemiconductor film 340 exposed. Therefore, as a method for adding thedopant 150, plasma treatment can be used as in Embodiment 2. Note thatthe plasma treatment is similar to the plasma treatment described inEmbodiment 2.

In the case where portions of the oxide semiconductor film 340 to be thepair of third regions 309 a and 309 b are exposed as in the transistor300, if a rare gas element is added as the dopant by plasma treatment,the portions to be the pair of third regions 309 a and 309 b might beetched and decreased in thickness as described in Embodiment 1.Therefore, in the case where the portions of the oxide semiconductorfilm 340 to be the pair of third regions 309 a and 309 b are exposed,hydrogen is preferably used as the dopant.

Even in the case where the gate insulating film 311 is also providedover the third regions 309 a and 309 b (and further over the secondelectrode 319 a and the third electrode 319 b) as in Embodiment 1, thetreatment for adding the dopant 150 to the oxide semiconductor film 340can be performed. In that case, the dopant 150 is added to the oxidesemiconductor film 340 through the gate insulating film 311 and thesidewall insulating films 315 a and 315 b. In the case of such astructure, a rare gas element can be used as the dopant 150 without anyproblem.

Note that the treatment for adding the dopant 150 to the oxidesemiconductor film 340 can be performed plural times as in Embodiment 1.

Further, heat treatment may be performed after the dopant 150 is added.The heat treatment may be performed in a manner similar to that of heattreatment performed in the formation of the oxide semiconductor film340, and is preferably performed at a temperature at which the pair ofsecond regions 307 a and 307 b and the pair of third regions 309 a and309 b are not crystallized.

Next, the interlayer insulating film 317 is formed over the firstelectrode 313, the second electrode 319 a, the third electrode 319 b,and the sidewall insulating films 315 a and 315 b in a manner similar tothat of the interlayer insulating film 117 described in Embodiment 1.Through the above steps, the transistor 300 can be manufactured (seeFIG. 6B).

The transistor 300 described in this embodiment can achieve an effectsimilar to that in Embodiment 1. Note that this embodiment can becombined with any of the other embodiments as appropriate.

Embodiment 4

In this embodiment, as for the transistors described in Embodiments 1 to3, influences of electric characteristics of the first region, the pairof second regions, and the pair of third regions which are included inthe oxide semiconductor film on the transistor will be described usingband diagrams. Note that the transistor 300 illustrated in FIGS. 6A and6B will be used as an example.

FIGS. 8A and 8B are each an energy band diagram (a schematic diagram) ofthe transistor 300 (see FIG. 6B) in a cross section along G-H. Note thatFIG. 8B shows the case where the potential of a source region is equalto that of a drain region (VD=0 V). The transistor 300 includes theoxide semiconductor film 303 including the first region 305 (denoted byOS1), the pair of second regions 307 a and 307 b (denoted by OS2), andthe pair of third regions 309 a and 309 b (denoted by OS3); and thesecond and third electrodes 319 a and 319 b (denoted by metal).

The channel formation region of the transistor 300 is formed using OS1,and OS1 is formed using an intrinsic (i-type) oxide semiconductor or asubstantially intrinsic oxide semiconductor which is obtained byremoving or eliminating impurities such as moisture (including hydrogen)from the film as much as possible so that the purity is increased. Thus,the Fermi level (Ef) can be at the same level as the intrinsic Fermilevel (Ei).

The low-concentration regions of the transistor 300 are formed usingOS2, and the source region and the drain region are formed using OS3. Asin the case of OS1, each of OS2 and OS3 is formed using an intrinsic(i-type) or substantially intrinsic oxide semiconductor which isobtained by removing or eliminating impurities such as moisture(including hydrogen) from the film as much as possible so that thepurity is increased. Then, hydrogen or one or more elements selectedfrom rare gas elements are added as a dopant to OS2 and OS3 so as tofunction as a donor or cause an oxygen vacancy. Accordingly, OS2 and OS3each have high carrier density and a Fermi level close to the conductionband, as compared with OS1.

FIG. 8A shows the relation between the vacuum level (denoted by Evac)and band structures of the first region 305 (denoted by OS1), the pairof second regions 307 a and 307 b (denoted by OS2), the pair of thirdregions 309 a and 309 b (denoted by OS3), and the second and thirdelectrodes 319 a and 319 b (denoted by metal). Here, IP represents theionization potential; Ea, the electron affinity; Eg, the band gap; andWf, the work function. In addition, Ec represents the conduction bandminimum; Ev, the valence band maximum; and Ef, the Fermi level. As for asign at the end of each symbol, 1 denotes OS1, 2 denotes OS2, 3 denotesOS3, and m denotes metal. Here, a metal having a work function of 4.1 eV(such as titanium) is assumed as metal.

OS1 is a highly purified oxide semiconductor and thus has extremely lowcarrier density; therefore, Ef_1 is around the middle point between Ecand Ev. Further, OS2 and OS3 are each an oxide semiconductor having highcarrier density, to which a dopant is added, and Ec_2 and Ec_3 generallycorrespond to Ef_2 and Ef_3, respectively. Each of the oxidesemiconductors denoted by OS1, OS2, and OS3 is said to have a band gap(Eg) of 3.15 eV and an electron affinity (Ea) of 4.3 eV.

As illustrated in FIG. 8B, when OS1 serving as the channel formationregion is in contact with OS2 serving as the low-concentration regions,carriers transfer so that the Fermi levels of OS1 and OS2 become equaland band edges of OS1 and OS2 are curved. Similarly, when OS2 serving asthe low-concentration regions is in contact with OS3 serving as thesource region and the drain region, carriers transfer so that the Fermilevels of OS2 and OS3 become equal and band edges of OS2 and OS3 arecurved. Further, similarly, when OS3 serving as the source region andthe drain region is in contact with metal, carriers transfer so that theFermi levels of OS3 and metal become equal and band edges of OS3 arecurved.

As described above, OS2 and OS3 which are oxide semiconductors havingdifferent high carrier densities are formed between OS1 serving as thechannel and metal serving as the second electrode 319 a and the thirdelectrode 319 b, whereby an ohmic contact can be formed between theoxide semiconductor film 303 and metal, and the contact resistance canbe reduced. As a result, the on-state current of the transistor 300 canbe increased. Moreover, the curve at the band edges of OS1 can besuppressed, and thus a short-channel effect in the transistor 300 can bereduced.

Embodiment 5

In this embodiment, examples of transistors which are different from thetransistors described in the above embodiments will be described withreference to FIGS. 9A to 9D.

FIG. 9A is a cross-sectional view of a transistor 400, and FIG. 9B is anenlarged view of a portion surrounded by a dotted line in FIG. 9A.

The transistor 400 has the following structure. A base insulating film402 is provided over a substrate 401. An oxide semiconductor film 403including a first region 405, a pair of second regions 407 a and 407 b,a pair of third regions 409 a and 409 b, and a pair of fourth regions410 a and 410 b is provided over the base insulating film 402. A secondelectrode 419 a and a third electrode 419 b are provided over the pairof fourth regions 410 a and 410 b. A gate insulating film 411 isprovided over the first region 405, the pair of second regions 407 a and407 b, the pair of third regions 409 a and 409 b, the pair of fourthregions 410 a and 410 b, the second electrode 419 a, and the thirdelectrode 419 b. A first electrode 413 is provided over the gateinsulating film 411 to overlap with the first region 405.

The transistor 400 is a top-gate top-contact transistor and is differentfrom the transistor 100, the transistor 200, and the transistor 300 inthat the pair of fourth regions 410 a and 410 b is provided.

The substrate 401, the base insulating film 402, the first region 405,the gate insulating film 411, the first electrode 413, the secondelectrode 419 a, and the third electrode 419 b can be formed in mannerssimilar to those of the substrate 101, the base insulating film 102, thefirst region 105, the gate insulating film 111, the first electrode 113,the second electrode 119 a, and the third electrode 119 b described inEmbodiment 1.

The first region 405 serving as a channel formation region is the CAACoxide semiconductor region described in Embodiment 1, and the pair offourth regions 410 a and 410 b also is the CAAC oxide semiconductorregion described in Embodiment 1. The pair of second regions 407 a and407 b and the pair of third regions 409 a and 409 b are each anamorphous region containing a dopant, and the dopant is similar to thatdescribed in Embodiment 1. Further, the dopant concentration of the pairof second regions 407 a and 407 b is different from the dopantconcentration of the pair of third regions 409 a and 409 b. The dopantconcentrations of the pair of second regions 407 a and 407 b and thepair of third regions 409 a and 409 b are within the respective rangesof dopant concentrations described in Embodiment 1.

In the transistor 400, after formation of the oxide semiconductor film140 described in Embodiment 1, regions having different dopantconcentrations (the first region 405, the pair of second regions 407 aand 407 b, the pair of third regions 409 a and 409 b, and the pair offourth regions 410 a and 410 b) can be formed by utilizing the firstelectrode 413, the second electrode 419 a, and the third electrode 419b.

The pair of third regions 409 a and 409 b is formed owing to taperedshapes of the second electrode 419 a and the third electrode 419 b. Inaddition, by reducing the thicknesses of the second electrode 419 a andthe third electrode 419 b, the area of the pair of third regions 409 aand 409 b can be increased.

Note that each of the transistor 100, the transistor 200, and thetransistor 300 is a transistor in which the regions having differentdopant concentrations (the first region, the pair of second regions, andthe pair of third regions) are formed by utilizing the first electrodeand the sidewall insulating film.

As described above, in the transistor 400, the pair of second regions407 a and 407 b and the pair of third regions 409 a and 409 b, whichhave different dopant concentrations, are provided with the first region405 serving as the channel formation region positioned in the middle;therefore, an electric field applied to the first region 405 serving asthe channel formation region can be relieved and thus a short-channeleffect can be suppressed.

Besides the transistor 400, a transistor 500 will be described as anexample of a transistor which is different from the transistorsdescribed in the above embodiments.

FIG. 9C is a cross-sectional view of the transistor 500, and FIG. 9D isan enlarged view of a portion surrounded by a dotted line in FIG. 9C.

The transistor 500 has the following structure. The base insulating film402 is provided over the substrate 401. The first electrode 413 and thegate insulating film 411 which covers the first electrode 413 areprovided over the base insulating film 402. The oxide semiconductor film403 including the first region 405, the pair of second regions 407 a and407 b, the pair of third regions 409 a and 409 b, and the pair of fourthregions 410 a and 410 b is provided over the gate insulating film 411.The second electrode 419 a and the third electrode 419 b are providedover the pair of fourth regions 410 a and 410 b. An insulating film 420is provided over the first region 405.

The transistor 500 is a bottom-gate top-contact transistor and isdifferent from the transistor 100, the transistor 200, and thetransistor 300 in that the pair of fourth regions 410 a and 410 b isprovided.

The substrate 401, the base insulating film 402, the first region 405,the gate insulating film 411, the first electrode 413, the secondelectrode 419 a, and the third electrode 419 b can be formed in mannerssimilar to those of the substrate 101, the base insulating film 102, thefirst region 105, the gate insulating film 111, the first electrode 113,the second electrode 119 a, and the third electrode 119 b described inEmbodiment 1. Since the transistor 500 has a bottom-gate structure, thefirst electrode 413 preferably has a tapered shape as in the case of thesecond electrode 419 a and the third electrode 419 b. The firstelectrode 413 is formed to have a tapered shape, whereby the coveragewith the gate insulating film 411 can be improved.

The first region 405 serving as a channel formation region is the CAACoxide semiconductor region described in Embodiment 1, and the pair offourth regions 410 a and 410 b also is the CAAC oxide semiconductorregion described in Embodiment 1. The pair of second regions 407 a and407 b and the pair of third regions 409 a and 409 b are each anamorphous region containing a dopant, and the dopant is similar to thatdescribed in Embodiment 1. Further, the dopant concentration of the pairof second regions 407 a and 407 b is different from the dopantconcentration of the pair of third regions 409 a and 409 b. The dopantconcentrations of the pair of second regions 407 a and 407 b and thepair of third regions 409 a and 409 b are within the respective rangesof dopant concentrations described in Embodiment 1.

In the transistor 500, after the oxide semiconductor film 140 describedin Embodiment 1 is formed over the gate insulating film 411, regionshaving different dopant concentrations (the first region 405, the pairof second regions 407 a and 407 b, the pair of third regions 409 a and409 b, and the pair of fourth regions 410 a and 410 b) can be formed byutilizing the second electrode 419 a, the third electrode 419 b, and theinsulating film 420. It is necessary that the insulating film 420 beformed to have a thickness large enough to prevent the dopant from beingadded to the first region 405.

Further, the pair of third regions 409 a and 409 b is formed owing totapered shapes of the second electrode 419 a and the third electrode 419b. In addition, by reducing the thicknesses of the second electrode 419a and the third electrode 419 b, the area of the pair of third regions409 a and 409 b can be increased.

Note that each of the transistor 100, the transistor 200, and thetransistor 300 is a transistor in which the regions having differentdopant concentrations (the first region, the pair of second regions, andthe pair of third regions) are formed by utilizing the first electrodeand the sidewall insulating film.

As described above, in the transistor 500, the pair of second regions407 a and 407 b and the pair of third regions 409 a and 409 b, whichhave different dopant concentrations, are provided with the first region405 serving as the channel formation region positioned in the middle;therefore, an electric field applied to the first region 405 serving asthe channel formation region can be relieved and thus a short-channeleffect can be suppressed.

Embodiment 6

In this embodiment, resistor elements each including an oxidesemiconductor to which a dopant is added will be described withreference to FIGS. 10A and 10B.

FIG. 10A illustrates a resistor element 600. The structure of theresistor element 600 will be described below. A base insulating film 602is provided over a substrate 601. An oxide semiconductor film 603 towhich a dopant is added is provided over the base insulating film 602.Conductive films 604 a and 604 b are provided over the oxidesemiconductor film 603. That is, the oxide semiconductor film 603 servesas a resistor in the resistor element 600. The oxide semiconductor film603 to which the dopant is added can be formed, for example, in such amanner that a portion where the gate insulating film 211 and the firstelectrode 213 are not formed over the oxide semiconductor film 140described in Embodiment 2 (see FIGS. 5A and 5B) is prepared and then thedopant is added to the portion. The conductive films 604 a and 604 b canbe formed using a conductive material which can be used for the firstelectrodes described in the above embodiments.

FIG. 10B illustrates a resistor element 610. The structure of theresistor element 610 will be described below. The base insulating film602 is provided over the substrate 601. The oxide semiconductor film 603to which a dopant is added is provided over the base insulating film602. An insulating film 606 is provided over the oxide semiconductorfilm 603. The conductive films 604 a and 604 b are provided in contactwith the insulating film 606 and part of the oxide semiconductor film603. The oxide semiconductor film 603 serves as a resistor also in theresistor element 610. The oxide semiconductor film 603 to which thedopant is added can be formed, for example, in such a manner that aportion where the gate insulating film 211 and the first electrode 213are not formed over the oxide semiconductor film 140 described inEmbodiment 2 (see FIGS. 5A and 5B) is prepared and then the dopant isadded to the portion. As the insulating film 606, any of the baseinsulating films, the gate insulating films, and the interlayerinsulating films described in the above embodiments may be used asappropriate. The conductive films 604 a and 604 b can be formed using aconductive material which can be used for the first electrodes describedin the above embodiments. In this manner, in the resistor element 610, acurrent path in the oxide semiconductor film 603 which serves as aresistor and is in contact with the conductive films 604 a and 604 b canbe uniform and more accurate resistance can be obtained.

Embodiment 7

An example of a circuit diagram of a memory element (hereinafter alsoreferred to as a memory cell) included in a semiconductor device isillustrated in FIG. 11A. The memory cell includes a transistor 1160 inwhich a channel formation region is formed using a material other thanan oxide semiconductor and a transistor 1162 in which a channelformation region is formed using an oxide semiconductor.

The transistor 1162 in which the channel formation region is formedusing an oxide semiconductor can be manufactured in accordance with anyof the above embodiments.

As illustrated in FIG. 11A, a gate electrode of the transistor 1160 iselectrically connected to one of a source electrode and a drainelectrode of the transistor 1162. A first wiring (a 1st Line, alsoreferred to as a source line) is electrically connected to a sourceelectrode of the transistor 1160. A second wiring (a 2nd Line, alsoreferred to as a bit line) is electrically connected to a drainelectrode of the transistor 1160. A third wiring (a 3rd Line, alsoreferred to as a first signal line) is electrically connected to theother of the source electrode and the drain electrode of the transistor1162. A fourth wiring (a 4th Line, also referred to as a second signalline) is electrically connected to a gate electrode of the transistor1162.

The transistor 1160 in which the channel formation region is formedusing a material other than an oxide semiconductor, e.g., single crystalsilicon can operate at sufficiently high speed. Therefore, with the useof the transistor 1160, high-speed reading of stored contents and thelike are possible. The transistor 1162 in which the channel formationregion is formed using an oxide semiconductor is characterized by itsoff-state current which is smaller than the off-state current of thetransistor 1160. Therefore, when the transistor 1162 is turned off, apotential of the gate electrode of the transistor 1160 can be held for avery long time.

By utilizing a characteristic in which the potential of the gateelectrode of the transistor 1160 can be held, writing, holding, andreading of data are possible as described below.

First, writing and holding of data will be described. First, a potentialof the fourth wiring is set to a potential at which the transistor 1162is turned on, so that the transistor 1162 is turned on. Thus, apotential of the third wiring is supplied to the gate electrode of thetransistor 1160 (writing). After that, the potential of the fourthwiring is set to a potential at which the transistor 1162 is turned off,so that the transistor 1162 is turned off, and thus, the potential ofthe gate electrode of the transistor 1160 is held (holding).

Since the off-state current of the transistor 1162 is smaller than theoff-state current of the transistor 1160, the potential of the gateelectrode of the transistor 1160 is held for a long time. For example,when the potential of the gate electrode of the transistor 1160 is apotential at which the transistor 1160 is in an on state, the on stateof the transistor 1160 is held for a long time. In addition, when thepotential of the gate electrode of the transistor 1160 is a potential atwhich the transistor 1160 is an off state, the off state of thetransistor 1160 is held for a long time.

Then, reading of data will be described. When a predetermined potential(a low potential) is supplied to the first wiring in a state where theon state or the off state of the transistor 1160 is held as describedabove, a potential of the second wiring varies depending on the on stateor the off state of the transistor 1160. For example, when thetransistor 1160 is in the on state, the potential of the second wiringbecomes lower than the potential of the first wiring. On the other hand,when the transistor 1160 is in the off state, the potential of thesecond wiring is not changed.

In such a manner, the potential of the second wiring and thepredetermined potential are compared with each other in a state wheredata is held, whereby the data can be read out.

Then, rewriting of data will be described. Rewriting of data isperformed in a manner similar to that of the writing and holding ofdata. That is, a potential of the fourth wiring is set to a potential atwhich the transistor 1162 is turned on, so that the transistor 1162 isturned on. Thus, a potential of the third wiring (a potential for newdata) is supplied to the gate electrode of the transistor 1160. Afterthat, the potential of the fourth wiring is set to a potential at whichthe transistor 1162 is turned off, so that the transistor 1162 is turnedoff, and thus, the new data is held.

In the memory cell according to the disclosed invention, data can bedirectly rewritten by another writing of data as described above. Forthat reason, erasing operation which is necessary for a flash memory orthe like is not needed, so that decrease in operation speed because oferasing operation can be suppressed. In other words, high-speedoperation of the semiconductor device including the memory cell can berealized.

FIG. 11B is a circuit diagram illustrating an application example of thememory cell illustrated in FIG. 11A.

A memory cell 1100 illustrated in FIG. 11B includes a first wiring SL (asource line), a second wiring BL (a bit line), a third wiring S1 (afirst signal line), a fourth wiring S2 (a second signal line), a fifthwiring WL (a word line), a transistor 1164 (a first transistor), atransistor 1161 (a second transistor), and a transistor 1163 (a thirdtransistor). In each of the transistors 1164 and 1163, a channelformation region is formed using a material other than an oxidesemiconductor, and in the transistor 1161, a channel formation region isformed using an oxide semiconductor.

Here, a gate electrode of the transistor 1164 is electrically connectedto one of a source electrode and a drain electrode of the transistor1161. In addition, the first wiring SL is electrically connected to asource electrode of the transistor 1164, and a drain electrode of thetransistor 1164 is electrically connected to a source electrode of thetransistor 1163. The second wiring BL is electrically connected to adrain electrode of the transistor 1163, and the third wiring S1 iselectrically connected to the other of the source electrode and thedrain electrode of the transistor 1161. The fourth wiring S2 iselectrically connected to a gate electrode of the transistor 1161, andthe fifth wiring WL is electrically connected to a gate electrode of thetransistor 1163.

Next, operation of the circuit will be specifically described.

When data is written into the memory cell 1100, the first wiring SL isset to 0 V, the fifth wiring WL is set to 0 V, the second wiring BL isset to 0 V, and the fourth wiring S2 is set to 2 V. The third wiring S1is set to 2 V in order to write data “1” and set to 0 V in order towrite data “0”. At this time, the transistor 1163 is in an off state andthe transistor 1161 is in an on state. Note that, to finish writing,before the potential of the third wiring S1 is changed, the fourthwiring S2 is set to 0 V so that the transistor 1161 is turned off.

As a result, a potential of a node (referred to as a node A) connectedto the gate electrode of the transistor 1164 is set to approximately 2 Vafter the writing of data “1” and set to approximately 0 V after thewriting of data “0”. Electric charge corresponding to a potential of thethird wiring S1 is accumulated at the node A; since the off-statecurrent of the transistor 1161 is smaller than that of a transistor inwhich a channel formation region is formed using single crystal silicon,the potential of the gate electrode of the transistor 1164 is held for along time.

When data is read from the memory cell, the first wiring SL is set to 0V, the fifth wiring WL is set to 2 V, the fourth wiring S2 is set to 0V, the third wiring S1 is set to 0 V, and a reading circuit connected tothe second wiring BL is set in an operation state. At this time, thetransistor 1163 is in an on state and the transistor 1161 is in an offstate.

The transistor 1164 is in an off state when data “0” has been written,that is, the node A is set to approximately 0 V, so that the resistancebetween the second wiring BL and the first wiring SL is high. On theother hand, the transistor 1164 is in an on state when data “1” has beenwritten, that is, the node A is set to approximately 2 V, so that theresistance between the second wiring BL and the first wiring SL is low.The reading circuit can read data “0” or data “1” in accordance with thedifference in resistance state of the memory cell. The second wiring BLat the time of the writing is set to 0 V; however, it may be in afloating state or may be charged to have a potential higher than 0 V.The third wiring S1 at the time of the reading is set to 0 V; however,it may be in a floating state or may be charged to have a potentialhigher than 0 V.

Note that data “1” and data “0” are defined for convenience and can bereversed. In addition, the above operation voltages are examples. Theoperation voltages are set so that the transistor 1164 is turned off inthe case of data “0” and turned on in the case of data “1”, thetransistor 1161 is turned on at the time of writing and turned off inperiods except the time of writing, and the transistor 1163 is turned onat the time of reading. A power supply potential VDD of a peripherallogic circuit may also be used instead of 2 V.

In this embodiment, the memory cell with a minimum storage unit (onebit) is described for easy understanding; however, the structure of thememory cell is not limited thereto. It is also possible to make a moredeveloped semiconductor device with a plurality of memory cellsconnected to each other as appropriate. For example, it is possible tomake a NAND-type or NOR-type semiconductor device by using more than oneof the above memory cells. The wiring structure is not limited to thatin FIG. 11A or 11B and can be changed as appropriate.

FIG. 12 is a block circuit diagram of a semiconductor device accordingto one embodiment of the present invention. The semiconductor device hasm×n bits of memory capacity.

The semiconductor device illustrated in FIG. 12 includes m fourthwirings S2(1) to S2(m), m fifth wirings WL(1) to WL(m), n second wiringsBL(1) to BL(n), n third wirings S1(1) to S1(n), a memory cell array 1110in which a plurality of memory cells 1100(1,1) to 1100(m,n) is arrangedin a matrix of m rows by n columns (m and n are each a natural number),and peripheral circuits such as a driver circuit 1111 for the secondwirings and the third wirings, a driver circuit 1113 for the fourthwirings and the fifth wirings, and a reading circuit 1112. A refreshcircuit or the like may be provided as another peripheral circuit.

A memory cell 1100(i,j) is considered as a typical example of the memorycell. Here, the memory cell 1100(i,j) (i is an integer greater than orequal to 1 and less than or equal to m and j is an integer greater thanor equal to 1 and less than or equal to n) is connected to a secondwiring BL(j), a third wiring S1(j), a fourth wiring S2(i), a fifthwiring WL(i), and a first wiring. A first wiring potential Vs issupplied to the first wiring. The second wirings BL(1) to BL(n) and thethird wirings S1(1) to S1(n) are connected to the driver circuit 1111for the second wirings and the third wirings and the reading circuit1112. The fifth wirings WL(1) to WL(m) and the fourth wirings S2(1) toS2(m) are connected to the driver circuit 1113 for the fourth wiringsand the fifth wirings.

The operation of the semiconductor device illustrated in FIG. 12 will bedescribed. In this structure, data is written and read per row.

When data is written into memory cells 1100(i,1) to 1100(i,n) of an i-throw, the first wiring potential Vs is set to 0 V, the fifth wiring WL(i)is set to 0 V, the second wirings BL(1) to BL(n) are set to 0 V, and thefourth wiring S2(i) is set to 2 V. At this time, the transistors 1161are turned on. Among the third wirings S1(1) to S1(n), the third wiringin a column in which data “1” is to be written is set to 2 V and thethird wiring in a column in which data “0” is to be written is set to 0V. Note that, to finish writing, the fourth wiring S2(i) is set to 0 Vbefore the potentials of the third wirings S1(1) to S1(n) are changed,so that the transistors 1161 are turned off. Moreover, a non-selectedfifth wiring WL and a non-selected fourth wiring S2 are set to 0 V.

As a result, the potential of the node (referred to as the node A)connected to the gate electrode of the transistor 1164 in the memorycell into which data “1” has been written is set to approximately 2 V,and the potential of the node A in the memory cell into which data “0”has been written is set to approximately 0 V (see FIG. 11B and FIG. 12). The potential of the node A of the non-selected memory cell is notchanged.

When data is read from the memory cells 1100(i,1) to 1100(i,n) of thei-th row, the first wiring potential Vs is set to 0 V, the fifth wiringWL(i) is set to 2 V, the fourth wiring S2(i) is set to 0 V, the thirdwirings S1(1) to S1(n) are set to 0 V, and the reading circuit connectedto the second wirings BL(1) to BL(n) is set in an operation state. Thereading circuit can read data “0” or data “1” in accordance with thedifference in resistance state of the memory cell, for example. Notethat the non-selected fifth wiring WL and the non-selected fourth wiringS2 are set to 0 V. The second wiring BL at the time of the writing isset to 0 V; however, it may be in a floating state or may be charged tohave a potential higher than 0 V. The third wiring S1 at the time of thereading is set to 0 V; however, it may be in a floating state or may becharged to have a potential higher than 0 V.

Note that data “1” and data “0” are defined for convenience and can bereversed. In addition, the above operation voltages are examples. Theoperation voltages are set so that the transistor 1164 is turned off inthe case of data “0” and turned on in the case of data “1”, thetransistor 1161 is turned on at the time of writing and turned off inperiods except the time of writing, and the transistor 1163 is turned onat the time of reading. A power supply potential VDD of a peripherallogic circuit may also be used instead of 2 V.

Embodiment 8

In this embodiment, an example of a circuit diagram of a memory cellincluding a capacitor will be shown. A memory cell 1170 illustrated inFIG. 13A includes a first wiring SL, a second wiring BL, a third wiringS1, a fourth wiring S2, a fifth wiring WL, a transistor 1171 (a firsttransistor), a transistor 1172 (a second transistor), and a capacitor1173. In the transistor 1171, a channel formation region is formed usinga material other than an oxide semiconductor, and in the transistor1172, a channel formation region is formed using an oxide semiconductor.

Here, a gate electrode of the transistor 1171, one of a source electrodeand a drain electrode of the transistor 1172, and one electrode of thecapacitor 1173 are electrically connected to each other. In addition,the first wiring SL and a source electrode of the transistor 1171 areelectrically connected to each other. The second wiring BL and a drainelectrode of the transistor 1171 are electrically connected to eachother. The third wiring S1 and the other of the source electrode and thedrain electrode of the transistor 1172 are electrically connected toeach other. The fourth wiring S2 and agate electrode of the transistor1172 are electrically connected to each other. The fifth wiring WL andthe other electrode of the capacitor 1173 are electrically connected toeach other.

Next, operation of the circuit will be specifically described.

When data is written into the memory cell 1170, the first wiring SL isset to 0 V, the fifth wiring WL is set to 0 V, the second wiring BL isset to 0 V, and the fourth wiring S2 is set to 2 V. The third wiring S1is set to 2 V in order to write data “1” and set to 0 V in order towrite data “0”. At this time, the transistor 1172 is turned on. Notethat, to finish writing, the fourth wiring S2 is set to 0 V before thepotential of the third wiring S1 is changed, so that the transistor 1172is turned off.

As a result, the potential of a node (referred to as a node A) connectedto the gate electrode of the transistor 1171 is set to approximately 2 Vafter the writing of data “1” and set to approximately 0 V after thewriting of data “0”.

When data is read from the memory cell 1170, the first wiring SL is setto 0 V, the fifth wiring WL is set to 2 V, the fourth wiring S2 is setto 0 V, the third wiring S1 is set to 0 V, and a reading circuitconnected to the second wiring BL is set in an operation state. At thistime, the transistor 1172 is turned off.

The state of the transistor 1171 in the case where the fifth wiring WLis set to 2 V will be described. The potential of the node A whichdetermines the state of the transistor 1171 depends on capacitance C1between the fifth wiring WL and the node A, and capacitance C2 betweenthe gate electrode of the transistor 1171 and the source and drainelectrodes of the transistor 1171.

Note that the third wiring S1 at the time of the reading is set to 0 V;however, it may be in a floating state or may be charged to have apotential higher than 0 V. Data “1” and data “0” are defined forconvenience and can be reversed.

The potential of the third wiring S1 at the time of writing may beselected from the potentials of data “0” and data “1” so that thetransistor 1172 is turned off after the writing and the transistor 1171is in an off state in the case where the potential of the fifth wiringWL is set to 0 V. The potential of the fifth wiring WL at the time ofreading is set so that the transistor 1171 is turned off in the case ofdata “0” and turned on in the case of data “1”. Furthermore, thethreshold voltage of the transistor 1171 is an example. The transistor1171 can have any threshold voltage so that the transistor 1171 canoperate in the above-described manner.

An example of a NOR-type semiconductor memory device in which a memorycell including a capacitor and a selection transistor having a firstgate electrode and a second gate electrode is used will be describedwith reference to FIG. 13B.

A semiconductor device illustrated in FIG. 13B according to oneembodiment of the present invention includes a memory cell arrayincluding a plurality of memory cells arranged in a matrix of I rows (Iis a natural number of 2 or more) by J columns (J is a natural number).

The memory cell array illustrated in FIG. 13B includes a plurality ofmemory cells 1180 arranged in a matrix of i rows (i is a natural numberof 3 or more) by j columns (j is a natural number of 3 or more), i wordlines WL (word lines WL_1 to WL_i), i capacitor lines CL (capacitorlines CL_1 to CL_i), i gate lines BGL (gate lines BGL_1 to BGL_i), j bitlines BL (bit lines BL_1 to BL_j), and a source line SL.

Further, each of the plurality of memory cells 1180 (also referred to asa memory cell 1180(M,N) (note that N is a natural number greater than orequal to 1 and less than or equal to j and that M is a natural numbergreater than or equal to 1 and less than or equal to i)) includes atransistor 1181(M,N), a capacitor 1183(M,N), and a transistor 1182(M,N).

Note that in the semiconductor memory device, the capacitor includes afirst capacitor electrode, a second capacitor electrode, and adielectric layer overlapping with the first capacitor electrode and thesecond capacitor electrode. Electric charge is accumulated in thecapacitor in accordance with voltage applied between the first capacitorelectrode and the second capacitor electrode.

The transistor 1181(M,N) is an n-channel transistor, which has a sourceelectrode, a drain electrode, a first gate electrode, and a second gateelectrode. Note that in the semiconductor memory device in thisembodiment, the transistor 1181 does not necessarily need to be ann-channel transistor.

One of the source electrode and the drain electrode of the transistor1181(M,N) is connected to a bit line BL_N. The first gate electrode ofthe transistor 1181(MN) is connected to a word line WL_M. The secondgate electrode of the transistor 1181(M,N) is connected to a gate lineBGL_M. With the structure in which the one of the source electrode andthe drain electrode of the transistor 1181(M,N) is connected to the bitline BL_N, data can be selectively read from memory cells.

The transistor 1181(M,N) serves as a selection transistor in the memorycell 1180(M,N).

As the transistor 1181(M,N), a transistor in which a channel formationregion is formed using an oxide semiconductor can be used.

The transistor 1182(M,N) is a p-channel transistor. Note that in thesemiconductor memory device in this embodiment, the transistor 1182 doesnot necessarily need to be a p-channel transistor.

One of a source electrode and a drain electrode of the transistor1182(M,N) is connected to the source line SL. The other of the sourceelectrode and the drain electrode of the transistor 1182(M,N) isconnected to the bit line BL_N. A gate electrode of the transistor1182(M,N) is connected to the other of the source electrode and thedrain electrode of the transistor 1181(M,N).

The transistor 1182(M,N) serves as an output transistor in the memorycell 1180(M,N). As the transistor 1182(M,N), for example, a transistorin which a channel formation region is formed using single crystalsilicon can be used.

A first capacitor electrode of the capacitor 1183(M,N) is connected to acapacitor line CL_M. A second capacitor electrode of the capacitor1183(M,N) is connected to the other of the source electrode and thedrain electrode of the transistor 1181(M,N). Note that the capacitor1183(M,N) serves as a storage capacitor.

The voltages of the word lines WL_1 to WL_i are controlled by, forexample, a driver circuit including a decoder.

The voltages of the bit lines BL_1 to BL_j are controlled by, forexample, a driver circuit including a decoder.

The voltages of the capacitor lines CL_1 to CL_i are controlled by, forexample, a driver circuit including a decoder.

The voltages of the gate lines BGL_1 to BGL_i are controlled by, forexample, a gate line driver circuit.

The gate line driver circuit is formed using a circuit which includes adiode and a capacitor whose first capacitor electrode is electricallyconnected to an anode of the diode and the gate line BGL, for example.

By adjustment of the voltage of the second gate electrode of thetransistor 1181, the threshold voltage of the transistor 1181 can beadjusted. Accordingly, by adjustment of the threshold voltage of thetransistor 1181 functioning as a selection transistor, current flowingbetween the source electrode and the drain electrode of the transistor1181 in an off state can be made extremely small. Thus, a data retentionperiod in the memory circuit can be made longer. In addition, voltagenecessary for writing and reading data can be made lower than that of aconventional semiconductor device; thus, power consumption can bereduced.

Embodiment 9

In this embodiment, examples of a semiconductor device using thetransistor described in any of the above embodiments will be describedwith reference to FIGS. 14A and 14B.

FIG. 14A illustrates an example of a semiconductor device whosestructure corresponds to that of a so-called dynamic random accessmemory (DRAM). A memory cell array 1120 illustrated in FIG. 14A has astructure in which a plurality of memory cells 1130 is arranged in amatrix. Further, the memory cell array 1120 includes m first wirings andn second wirings. Note that in this embodiment, the first wiring and thesecond wiring are referred to as a bit line BL and a word line WL,respectively.

The memory cell 1130 includes a transistor 1131 and a capacitor 1132.Agate electrode of the transistor 1131 is connected to the first wiring(the word line WL). Further, one of a source electrode and a drainelectrode of the transistor 1131 is connected to the second wiring (thebit line BL). The other of the source electrode and the drain electrodeof the transistor 1131 is connected to one electrode of the capacitor.The other electrode of the capacitor is connected to a capacitor line CLand is supplied with a predetermined potential. The transistor describedin any of the above embodiments is applied to the transistor 1131.

The transistor in which a channel formation region is formed using anoxide semiconductor, which is described in any of the above embodiments,is characterized by having smaller off-state current than a transistorin which a channel formation region is formed using single crystalsilicon. Accordingly, when the transistor is applied to thesemiconductor device illustrated in FIG. 14A, which is regarded as aso-called DRAM, a substantially nonvolatile memory can be obtained.

FIG. 14B illustrates an example of a semiconductor device whosestructure corresponds to that of a so-called static random access memory(SRAM). A memory cell array 1140 illustrated in FIG. 14B can have astructure in which a plurality of memory cells 1150 is arranged in amatrix. Further, the memory cell array 1140 includes a plurality offirst wirings (word lines WL), a plurality of second wirings (bit linesBL), and a plurality of third wirings (inverted bit lines/BL).

The memory cell 1150 includes a first transistor 1151, a secondtransistor 1152, a third transistor 1153, a fourth transistor 1154, afifth transistor 1155, and a sixth transistor 1156. The first transistor1151 and the second transistor 1152 function as selection transistors.One of the third transistor 1153 and the fourth transistor 1154 is ann-channel transistor (here, the fourth transistor 1154 is an n-channeltransistor), and the other of the third transistor 1153 and the fourthtransistor 1154 is a p-channel transistor (here, the third transistor1153 is a p-channel transistor). In other words, the third transistor1153 and the fourth transistor 1154 form a CMOS circuit. Similarly, thefifth transistor 1155 and the sixth transistor 1156 form a CMOS circuit.

The first transistor 1151, the second transistor 1152, the fourthtransistor 1154, and the sixth transistor 1156 are n-channel transistorsand the transistor described in any of the above embodiments can beapplied to these transistors. Each of the third transistor 1153 and thefifth transistor 1155 is a p-channel transistor in which a channelformation region is formed using a material (e.g., single crystalsilicon) other than an oxide semiconductor.

The methods, structures, and the like described in this embodiment canbe combined with any of the methods, structures, and the like describedin the other embodiments, as appropriate.

Embodiment 10

A central processing unit (CPU) can be formed using a transistor inwhich a channel formation region is formed using an oxide semiconductorfor at least part of the CPU.

FIG. 15A is a block diagram illustrating a specific structure of a CPU.The CPU illustrated in FIG. 15A includes an arithmetic logic unit (ALU)1191, an ALU controller 1192, an instruction decoder 1193, an interruptcontroller 1194, a timing controller 1195, a register 1196, a registercontroller 1197, a bus interface (Bus I/F) 1198, a rewritable ROM 1199,and an ROM interface (ROM I/F) 1189 over a substrate 1190. Asemiconductor substrate, an SOI substrate, a glass substrate, or thelike is used as the substrate 1190. The ROM 1199 and the ROM I/F 1189may be provided over a separate chip. Obviously, the CPU illustrated inFIG. 15A is only an example in which the structure is simplified, and anactual CPU may have various structures depending on the application.

An instruction that is input to the CPU through the Bus I/F 1198 isinput to the instruction decoder 1193 and decoded therein, and then,input to the ALU controller 1192, the interrupt controller 1194, theregister controller 1197, and the timing controller 1195.

The ALU controller 1192, the interrupt controller 1194, the registercontroller 1197, and the timing controller 1195 conduct various controlsin accordance with the decoded instruction. Specifically, the ALUcontroller 1192 generates signals for controlling the operation of theALU 1191. While the CPU is executing a program, the interrupt controller1194 judges an interrupt request from an external input/output device ora peripheral circuit on the basis of its priority or a mask state, andprocesses the request. The register controller 1197 generates an addressof the register 1196, and reads/writes data from/into the register 1196in accordance with the state of the CPU.

The timing controller 1195 generates signals for controlling operationtimings of the ALU 1191, the ALU controller 1192, the instructiondecoder 1193, the interrupt controller 1194, and the register controller1197. For example, the timing controller 1195 includes an internal clockgenerator for generating an internal clock signal CLK2 based on areference clock signal CLK1, and supplies the clock signal CLK2 to theabove circuits.

In the CPU illustrated in FIG. 15A, a memory element is provided in theregister 1196. The memory element described in Embodiment 8 can be usedas the memory element provided in the register 1196.

In the CPU illustrated in FIG. 15A, the register controller 1197 selectsoperation of holding data in the register 1196 in accordance with aninstruction from the ALU 1191. That is, the register controller 1197selects whether data is held by a phase-inversion element or a capacitorin the memory element included in the register 1196. When data holdingby the phase-inversion element is selected, power supply voltage issupplied to the memory element in the register 1196. When data holdingby the capacitor is selected, the data is rewritten in the capacitor,and supply of power supply voltage to the memory element in the register1196 can be stopped.

The power supply can be stopped by providing a switching element betweena memory element group and a node to which a power supply potential VDDor a power supply potential VSS is supplied, as illustrated in FIG. 15Bor FIG. 15C. Circuits illustrated in FIGS. 15B and 15C will be describedbelow.

FIGS. 15B and 15C each illustrate an example of a structure of a memorycircuit including a transistor in which a channel formation region isformed using an oxide semiconductor as a switching element forcontrolling supply of a power supply potential to a memory element.

The memory device illustrated in FIG. 15B includes a switching element1141 and a memory element group 1143 including a plurality of memoryelements 1142. Specifically, as each of the memory elements 1142, thememory element described in Embodiment 8 can be used. Each of the memoryelements 1142 included in the memory element group 1143 is supplied withthe high-level power supply potential VDD via the switching element1141. Further, each of the memory elements 1142 included in the memoryelement group 1143 is supplied with a potential of a signal IN and thelow-level power supply potential VSS.

In FIG. 15B, a transistor in which a channel formation region is formedusing an oxide semiconductor is used as the switching element 1141, andthe switching of the transistor is controlled by a signal Sig A suppliedto a gate electrode thereof.

Note that FIG. 15B illustrates the structure in which the switchingelement 1141 includes only one transistor; however, without limitationthereto, the switching element 1141 may include a plurality oftransistors. In the case where the switching element 1141 includes aplurality of transistors which serves as switching elements, theplurality of transistors may be connected to each other in parallel, inseries, or in combination of parallel connection and series connection.

Although the switching element 1141 controls the supply of thehigh-level power supply potential VDD to each of the memory elements1142 included in the memory element group 1143 in FIG. 15B, theswitching element 1141 may control the supply of the low-level powersupply potential VSS.

In FIG. 15C, an example of a memory device in which each of the memoryelements 1142 included in the memory element group 1143 is supplied withthe low-level power supply potential VSS via the switching element 1141is illustrated. The supply of the low-level power supply potential VSSto each of the memory elements 1142 included in the memory element group1143 can be controlled by the switching element 1141.

When a switching element is provided between a memory element group anda node to which the power supply potential VDD or the power supplypotential VSS is supplied, data can be held even in the case whereoperation of a CPU is temporarily stopped and the supply of the powersupply voltage is stopped; accordingly, power consumption can bereduced. Specifically, for example, while a user of a personal computerdoes not input data to an input device such as a keyboard, the operationof the CPU can be stopped, so that the power consumption can be reduced.

Although the CPU is given as an example, the transistor can also beapplied to an LSI such as a digital signal processor (DSP), a customLSI, or a field programmable gate array (FPGA).

This embodiment can be combined with any of the above embodiments asappropriate.

This application is based on Japanese Patent Application serial no.2010-292895 filed with the Japan Patent Office on Dec. 28, 2010, theentire contents of which are hereby incorporated by reference.

What is claimed is:
 1. A semiconductor device comprising: a firsttransistor comprising: a first terminal electrically connected to afirst wiring; a second terminal electrically connected to a secondwiring; and a third terminal; a second transistor comprising: a fourthterminal functioning as a gate of the second transistor electricallyconnected to the third terminal; a fifth terminal electrically connectedto a third wiring; and a sixth terminal; a third transistor comprising:a seventh terminal electrically connected to the sixth terminal; aneighth terminal electrically connected to a fourth wiring; and a ninthterminal electrically connected to a fifth wiring; wherein one of thefirst transistor, the second transistor, and the third transistorcomprises: a first insulating layer; an oxide semiconductor film overthe first insulating layer comprising: a first region; a pair of secondregions, the first region located between the pair of second regions;and a pair of third regions, the first region and the pair of secondregions located between the pair of third regions; a gate insulatingfilm over the oxide semiconductor film; a gate electrode over the gateinsulating film and overlapping with the first region; a first electrodein contact with a top surface of the oxide semiconductor film; and asecond electrode in contact with the top surface of the oxidesemiconductor film, wherein the gate insulating film is in contact witha side surface of the oxide semiconductor film, a side surface of thefirst electrode, a side surface of the second electrode, and a topsurface of the first insulating layer, wherein the oxide semiconductorfilm comprises indium, gallium, and zinc, wherein the gate insulatingfilm extends beyond an end portion of the gate electrode and overlapsthe first region and the pair of second regions, wherein the gateinsulating film is not in contact with a side surface of the gateelectrode, wherein the first region comprises a crystalline portion,wherein the gate electrode, the first electrode, and the secondelectrode comprise copper, and wherein a conductivity of the pair ofthird regions is higher than a conductivity of the pair of secondregions.
 2. The semiconductor device according to claim 1, wherein thegate insulating film is an oxide insulating film.
 3. The semiconductordevice according to claim 1, wherein the one of the first transistor,the second transistor, and the third transistor is an n-channeltransistor.
 4. The semiconductor device according to claim 1, whereinthe gate insulating film overlaps with part of the pair of thirdregions.
 5. The semiconductor device according to claim 1, wherein abottom surface of an insulating film provided over the gate electrode isin contact with the gate insulating film.
 6. A semiconductor devicecomprising: a first transistor comprising: a first terminal electricallyconnected to a first wiring; a second terminal electrically connected toa second wiring; and a third terminal; a second transistor comprising: afourth terminal functioning as a gate of the second transistorelectrically connected to the third terminal; a fifth terminalelectrically connected to a third wiring; and a sixth terminal; a thirdtransistor comprising: a seventh terminal electrically connected to thesixth terminal; an eighth terminal electrically connected to a fourthwiring; and a ninth terminal electrically connected to a fifth wiring;wherein one of the first transistor, the second transistor, and thethird transistor comprises: a first insulating layer; an oxidesemiconductor film over the first insulating layer comprising: a firstregion; and a pair of second regions, the first region located betweenthe pair of second regions; a gate insulating film over the oxidesemiconductor film; a gate electrode over the gate insulating film andoverlapping with the first region; a first electrode in contact with atop surface of the oxide semiconductor film; and a second electrode incontact with the top surface of the oxide semiconductor film, whereinthe gate insulating film is in contact with a side surface of the oxidesemiconductor film, a side surface of the first electrode, a sidesurface of the second electrode, and a top surface of the firstinsulating layer, wherein the oxide semiconductor film comprises indium,gallium, and zinc, and wherein a conductivity of the pair of secondregions is higher than a conductivity of the first region.
 7. Thesemiconductor device according to claim 6, wherein the gate insulatingfilm overlaps with part of the pair of second regions.
 8. Thesemiconductor device according to claim 6, wherein a bottom surface ofan insulating film provided over the gate electrode is in contact withthe gate insulating film.